The first ghost changes appearance, the second ghost ages, and the third ghost is dark and vague.
The first ghost has a changing appearance that represents the variety of the past, and how we alter it in our memories. It is the Ghost of Christmas Past It looks simultaneously male and female, old and young. He has a light emanating from his head, and it seems to shimmer.
[The] figure itself fluctuated in its distinctness: being now a thing with one arm, now with one leg, now with twenty legs, now a pair of legs without a head, now a head without a body … (Ch. 2)
It wears a white tunic, and carries a branch of holly. This also represents the Christmas season. During the trip, the ghost is mostly gentle with Scrooge, but sometimes stern. Scrooge finally can’t take it anymore when he sees how happy his former fiancée Belle is with her new husband. Looking at the life he could have had, he puts out the light of the Ghost of Christmas Past. This ghost's goal was to show Scrooge that he once had people he cared about, and people who cared about him. He also shows him that his pain comes from pain in his past.
The Ghost of Christmas Present is jolly, perhaps serving as an inspiration for the modern day depiction of Santa Claus. Indeed, he wears a robe lined with white fur. Unlike Santa’s robe (whose red color was chosen by Coca-cola), the Ghost of Christmas Present’s robe is green. His hair and beard start out brown, but age to white as the day goes on because this ghost is only alive one day. This is definitely the jolliest of the ghosts.
Its dark brown curls were long and free; free as its genial face, its sparkling eye, its open hand, its cheery voice, its unconstrained demeanour, and its joyful air. (Ch. 3)
The ghost explains to Scrooge at the end of the day that he only lives for one day, and Scrooge has learned his lesson by now. His goal was mostly to show Scrooge that people do not need a lot of money to celebrate Christmas and enjoy celebrating it. They enjoy celebrating it with one another by being together.
The last ghost is the most frightening to Scrooge. He does not talk at all, and he looks like the Grim Reaper. He wears a black robe and seems to have skeletal hands.
There was something very awful, too, in the spectre's being provided with an infernal atmosphere of its own … for though the Ghost sat perfectly motionless, its hair, and skirts, and tassels, were still agitated as by the hot vapour from an oven. (Ch. 4)
Scrooge’s fear of the ghost represents his fear of the future. This ghost is as vague as he is because the future is unknown. This is why he has no clear features. This ghost's goal is to cement the lesson by reminding Scrooge what will happen to him if he does not change. Scrooge does not even recognize the future self he sees, because she already knows himself as a changed person.
Dickens gives each ghost distinct features to represent Scrooge’s steps in the journey, and the element of time that the ghost depicts. Scrooge interacts differently with each ghost, and the reader will have a different reaction to each ghost.