Color symbolism can be very provocative. Since novels are generally not illustrated, authors rely upon descriptions to create visual pictures, and color is a large part of that. It is an interesting contradiction and bit of dramatic irony that Lenina wears green against her social category. Green is often considered the color of an outsider or alien character (this can be seen in many cinematic moments; for example, the Wicked Witch of the West's green skin in The Wizard of Oz; or the bright green dress worn by Cyd Charisse as a femme fatale in Singin' in the Rain).
The shade of green used is important to its symbolism; bluer greens are more ethereal; greens with more yellow are more earthy. Olive greens can denote decay or illness; spring green denotes innocence and fertility. Without a clear picture of the type of green being described, it is safest to default to the most basic symbolic meanings of green: harmony, nature, balance. Certainly Lenina challenges the status quo, which suggests she potentially has a balancing influence.
There does seem to be some inconsistency here, as Lenina is a Beta Plus who does wear green. If we remember how the children in this future world are trained to accept their station in life, we can see that in theory at least each caste has its own colour that it wears religiously:
Alpha children wear grey. They work much harder than we do, because they're so frightfully clever... Gammas are stupid. They all wear green, and Delta children wear khaki...
Maybe we can conclude from this that Huxley merely got confused with his colours at various points in the novel or that Lenina, as a Beta Plus, wears different colours from the rest of normal Betas. Either way, do not let the colour of her clothing detract from the way that she is presented and viewed in this novel as a sex object, both by herself and others, which the opening chapters make very clear.