Just be totally breezy about it.
If asked: you think it'll look a bit crap, because greens are hard to match, green hair dye fades badly & needs re-doing a lot. If it doesn't match it'll look silly, whereas if she does somehow manage a perfect match with her frock on the day, well, that'll look even more ridiculous.
But hey, she's your mate, you didn't choose her as BM for her dress sense tolerant eye roll at this point.
If she's doing it to wind you up - it isn't, so she'll get the message that she's going to look a bit of a tit, probably not feature prominently in the photos, & she's generally being tiresome.
If she isn't - well, it's 3 months away. She'll have got bored with it & dyed it leopardskin purple by then!
Definitely not worth a Bridezilla, because no-one else will care. Worst case, she has green hair on the day, it loks a bit silly, & there'll be half a dozen photos knocking about in an album somewhere that your grandchildren will eventually get to giggle over!