castes is in place so that people can quickly and easily know what caste a person is in. The dystopian world described in Huxley's novel is very superficial, so of course they would use a superficial designation like color coding. It is surface-level symbolism, but also very effective psychologically. The world is also one that relies heavily on simplistic psychological conditioning, as seen with the Pavlovian type conditioning and the electric shocks. 

In such a world, it is incredibly important to know at all times who is in which caste, which the color system perpetuates and concretizes. It is vital for the continuation of the oppressive state because the caste system is the absolute most powerful tool the dystopian government, the World State, has to oppress and brainwash their citizens. Using genetic modification techniques, the World State makes the lower castes physically unable to rebel. In their fetal state, they are introduced to alcohol in their blood, deprived of oxygen, and then afterwards they must undergo extreme brainwashing and conditioning. Thus, lower class citizens are created to be servile and lesser. It is in their "nature," as manipulated as it is. This system allows for the dominance of certain people over others, and the systematic nature of the oppression makes it extremely difficult to challenge. In fact, one could argue that it's even impossible to challenge based on the way in which the oppression has taken root. So although it seems arbitrary, the color system in the World State castes is actually an incredibly systematic way of keeping the status quo. 

One significant difference is that the two highest castes - Beta and Alpha - are not mass-produced, and thus have a degree of individuality. That individuality extends, logically, to physical characteristics as well as to personality. Moreover, just as the highest caste - Alpha - is comprised of the tallest, most beautiful individuals, the Gamma, Delta, and Epsilon workers are progressively shorter and uglier.

Huxley is implying that there is a caste system today, though based on race and economics as opposed to something similar to cloning.

There are many differences, but briefly, the different castes are Alphas, Betas, Gammas, Deltas, and Epsilons. The upper castes are intelligent and have managerial jobs, whereas the lower castes do the manual labor. The Alphas have what we would consider the best jobs, and it continues down until the Epsilons, who have the least skilled jobs. The Alphas are tall and fair, while the Epsilons are dark-skinned, reflecting the prejudices of that time period. They also wear a specific color. For example, the Gammas wear green and the Deltas wear Khaki.

Simply put, the caste system in Brave New World is based on intelligence that was more or less determined during conception. 

Therefore, those of higher castes (and thus higher intellect), were the most nurtured throughout their embryonic development. However, those of lower castes had their mental development actively sabotaged by dividing one embryo multiple times through Bokanovsky's Process and then by adding alcohol or restricting oxygen flow to these fetuses. There are five total castes, all of which are characterized below and organized in descending order.

Alphas: They typically dress in gray and are extremely intelligent. They hold the most sophisticated jobs. They are prone to becoming very individual and to crave more than a strictly structured society can give them (e.g. Bernard and Helmholtz). 

Betas: They usually wear mulberry colored clothes and are intelligent enough to easily perform skilled work, though the work they do is slightly less complex than what Alphas do. As well, they are far less likely to be dissatisfied with what society has to offer them.

Gammas: They most often dress in green and are considered to be of average intelligence. They are moderately skilled laborers, often working as mechanics or in other positions that involve manual labor and a degree of thought. Gammas and the proceeding castes go through Bokanovsky's Process and have their development hindered as a fetus. 

Deltas: They dress in khaki and have very little individuality and intellect. They are employed in simple and repetitive professions such as helicopter attendants or factory workers.

Epsilons: They wear black and are often referred to as "semi-morons" because they lack even the ability to read and write. They do the most menial and unwanted jobs. 

Also, due to the sabotage of embryonic growth, people of lower castes are much shorter and considered uglier than people of higher castes.

It should be noted that all of the above plays into the overall theme of class conflict Huxley is trying to present and satirize.  

It becomes clear as you read the first chapter of this amazing book and we are given a tour of a "Hatchery" that there are five castes in this society, each named after the first five letters of the Greek alphabet: alpha, beta, gamma, delta, epsilon. It also becomes clear that they are organised in order of importance, with alphas being the most important and epsilons the least. Consider what the Director of Hatcheries says to his visitors:

"We also predestine and condition. We decant our babies as socialised human beings, as Alphas or Epsilons, as future sewage workers or future..." He was going to say future World Controllers, but correcting himself, said "future Directors of Hatcheries" instead.

As we read on it becomes clear that Epsilons are given less oxygen to ensure that they have an "Epsilon environment as well as an Epsilon heredity" ensuring that this society is not troubled by any intelligent Epsilons.

Thus the first chapter introduces us to the most radically stratified class system in the world, all organized by genetic engineering and advanced science, to create five types of humans with clearly demarcated positions and roles in society.

In Brave New World, the World State has created five castes of test-tube humans: the Alphas, Betas, Gammas, Deltas, and Epsilons (the Greek letters A through E), based primarily on intelligence level. Within each caste, there are higher and lower graduations: one can be, for example, an Alpha-plus or a Delta-minus.

Beyond sheer intelligence, other characteristics distinguish the castes, for example, height. The Alphas, who are the most intelligent, are also, in general, the tallest. In fact, much is made of the fact that Bernard Marx, though an Alpha, is somewhat short, with rumors flying that something went wrong with his test-tube. Deltas and Epsilons, who are the least intelligent castes, are also the shortest. They are also produced in bigger batches of twins than the higher castes, making them less individualized.

Finally, nowhere in the book do we find Alpha women, whereas all the Betas we meet are female: it is as if this society is designed to allow more intelligent men access for sex and companionship to a group of less intelligent women. All of this reflects the society in which Huxley lived: because of educational differences, well-educated ("intelligent") men tended to marry less well-educated ("less intelligent") women in early-twentieth-century Britain, and especially in Britain's Asian colonies because of lack of nutrition, the Asians doing most of the manual labor tended to be noticeably smaller than the Caucasians.

The purpose of the castes is to create a completely harmonious society. Everyone is supposed to be fitted to a job that matches their intelligence, and each caste is conditioned to be happy with its lot and to look with horror or disdain at the dress and habits of the superior and inferior castes, respectively.

The castes are distinguished from each other by size, intelligence, appearance, the color of clothing they wear and the jobs they are assigned to as adults.

Alphas = tall, beautiful, intelligent an wear grey clothing

Betas = Not as tall, but still pretty good looking and intelligent.  They wear white clothing.

Deltas = average height and looks, not as intelligent and wear khakis.

Gammas = "Gammas are stupid. They all wear green. . ." (31).

Epsilons = "They're too stupid to be able to read or write.  Besides they wear black, which is such a beastly coulour" (30).

The purpose of this class system is to keep everyone under control mentally and physically. People at the bottom of the classes are brain-washed to believe they are at the bottom for a good reason and to accept it. This keeps everyone in his or her own place in society in order for everyone to be happy; therefore, if everyone is happy, then no one will fight and be unhappy (thereby averting things like war).

There are a total of five castes in Aldous Huxley's futuristic, technologically advanced World State, where society and its citizens are manufactured to ensure stability, comfort, efficiency, and happiness. Each caste is identified by appearance, occupation, and intelligence. The Alphas are the elite caste of the World State and consist of the most physically attractive, intelligent, and powerful members of society. Alphas occupy the most important occupations in the World State and are influential, prominent citizens. They tend to be professors, scientists, and managers. The Betas are the second-highest caste in society and are primarily skilled workers. The Alphas wear gray, while the Betas wear mulberry or maroon. The three lowest castes of the World State are the Gammas, Deltas, and Epsilons. The Gammas are semi-skilled workers, who wear green, while the Deltas are low-skilled workers, who wear khaki. The Epsilons occupy the lowest caste and wear black. They are much shorter and less attractive than members of the other castes and occupy laborious, low-functioning positions. It is important to note that each manufactured citizen is specifically conditioned to enjoy and thrive in their particular caste in order to ensure stability and efficiency throughout society.

There are five castes in the novel. In descending order, they are named for the first five letters of the Greek alphabet, Alphas, Betas, Gammas, Deltas, and Epsilons. The upper castes are bred to be the intelligent part of society in positions of management. The lower castes are bred to do the menial tasks of society. Each caste is conditioned during their sleep to be happy with their caste and not want to be a part of any other caste.

In the New World of Aldous Huxley's novel, there is a caste system that originates from artificially contrived births in the Central London Hatchery and Conditioning Center.  These castes are assigned letters of the Greek alphabet in descending order of intelligence:  Alpha, Beta, Gamma, Delta, and Epsilon.  All but the Alphas and Betas undergo the Bokavonsky Process which involves shocking an egg so that it divides into ninety-six separate human beings.  These Gammas, Deltas, and Epsilons also have oxygen deprivation in order to dumb them down since they are designed for the lesser tasks of the society, such as operating machinery.  While they are children, these castes also receive sleep conditioning, or hypnopedia.  Betas, for instance, are taught to hate Gammas and Deltas, who wear green and khaki.  Lenina, a Beta-Minus who is a vaccinator at the Hatchery remarks that she hates khaki several times in the narrative.  Gammas are often machine-minders, butlers, and other moderately thinking jobs.  Epsilons, the darkest race, are the ones who do mindless tasks, such as operating elevators.  The Predestinators determine how many humans will be needed

Holding the managerial positions, the Alphas and Betas are also divided as Alpha-Pluses such as Helmholtz, who is the "emotional engineer" in charge of writing propaganda to keep people established in their proper beliefs, and Bernard Marx, who is a psychologist.Foster, also an Alpba, works closely under the Direcor of the hatchery; he is a contented Alpha.

The Beta pluses or minuses differ slightly from one another. Lenina and Fanny are good friends, and both suffer from lupus, a disease of the related young women.  In order control the masses, soma is handed to everyone.