I never quite understood why Lenina wears green if she is a beta, since betas don't wear green.
There are a couple of possibilities, some of which are also touched on in the answers to In Brave New World, what caste is Lenina Crowne? I'm listing them in what I think is decreasing order of probability.
The author made a mistake.
The colour associated with the Beta caste is only stated once in the whole story, in a throwaway line in Chapter IV (emphasis mine):
Like aphides and ants, the leaf-green Gamma girls, the black Semi-Morons swarmed round the entrances, or stood in queues to take their places in the monorail tram-cars. Mulberry-coloured Beta-Minuses came and went among the crowd. The roof of the main building was alive with the alighting and departure of helicopters.
'My word,' said Lenina, 'I'm glad I'm not a Gamma.'
Perhaps either this use of the word "mulberry" was a mistake, or the author forgot it when describing Lenina's green outfit (although the latter is mentioned more than once).
Betas have more freedom in what colours to wear.
Perhaps the higher castes are not so strictly required to wear only the appropriate colour for their caste. As a Beta, perhaps Lenina has a little more latitude in clothing colour - presumably so long as it's not the same colour as one associated with another caste, so a different type of green from the Gammas' "leaf-green" might be acceptable.
She's a Beta-Plus with a different associated colour from Beta-Minuses.
It's never mentioned explicitly which caste Lenina belongs to, although it can be more or less deduced that she is Beta. But whether that's Beta-Plus or Beta-Minus is another question, and it's just possible that the two may have different associated colours. The "mulberry" quote only refers to Beta-Minuses, after all. Possible but perhaps unlikely, since only one colour is mentioned for each of the other castes.
"Mulberry" referring to a green colour?
Two answerers on Quora seem to think "mulberry" means green, one of them saying "Lenina wears bottle green shorts, which is close to the mulberry green colour. Thus, the readers know she is Beta." and the other asserting that "While mulberry today indicates a deep red, there is a colour called “mulberry green" that's very close to old dark green bottles." However, I couldn't find any evidence that this is actually true: I could only find mulberry green as a brand of the company Mulberry, which definitely postdates Huxley. And there are other mistakes made by the answerers on that Quora page, so definitely take this with a pinch of salt.