"The color of the peach juice."
Translation:Το χρώμα του χυμού ροδάκινο.
This discussion has a lot of info on the matter: https://www.duolingo.com/comment/17530852 . Hope it answers both of your questions!! :) in case it's too much: here is the sum-up: when a noun characterizes another noun, it's most of the time ok to put it in genitive WITHOUT article. So, orange juice=Χυμός πορτοκάλι (an omitted από is between them) or χυμός πορτοκαλιού. So the characterized noun had whatever case is required, but the characterizing one is always in genitive/nominative (if a preposition is omitted). But of these different ways of putting the characterized noun, sometimes one of them is preferred (or maybe a compound word) so this will solve itself through learning.