I have wondered that myself. But I think what is going on is that not only each caste, but also each sub-caste has its own distinctive color. Also, please note that Gammas do not wear mulberry.
In Chapter 4, Lenina is with Bernard and they are watching the shift change at a factory. They are seeing all the people coming out. The people she sees who are wearing mulberry are all Beta Minuses.
So, what I have concluded is that Lenina is simply a Beta without a "Minus."
As to the Gammas and green and Lenina's clothes, I have two guesses. Either A) Huxley just lost track of his colors and no one noticed or B) her green clothes are a different shade than those worn by Gammas.
Lenina is a beta. People in the novel are divided into five castes, based primarily on intelligence. The alphas are the most intelligent caste, and as the Greek letters indicate, each caste descends in intelligence from alpha to beta to gamma to delta. Epsilons, who perform the simplest tasks in society, are at the bottom. Each caste is conditioned to accept its position in the hierarchy.
As a beta, Lenina has been taught to conform, to dislike being alone, and to consume. These messages have been reinforced through years of conditioning. Lenina has been conditioned to enjoy being a beta. Betas are expected to shop and buy new things constantly in order to keep factory production high and employment levels stable. Lenina, as beta, represents a typical shallow product of a materialist culture and becomes a comic foil to the Savage, John, who falls in love with her. Their two cultures clash: neither can understand the other and Lenina lacks the critical thinking skills needed to bridge the gap.
Lenina is a Beta plus. She is a foil for Linda, also a Beta (minus).
During the tour of the hatcheries we hear her voice in the background:
"… all wear green," said a soft but very distinct voice, beginning in the middle of a sentence, "and Delta Children wear khaki. Oh no, I don't want to play with Delta children. And Epsilons are still worse. They're too stupid to be able to read or write. Besides they wear black, which is such a beastly colour. I'm so glad I'm a Beta."