Both are correct and accepted :)
Χυμός πορτοκάλι is more common. The full phrase would be Χυμός με γεύση πορτοκάλι (Juice with orange flavor), but for brevity reasons με γεύση has been omitted.
From this point of view χυμός πορτοκαλιού should be the best translation, but it is not as common as χυμός πορτοκάλι.
This happens with many fruit, such as χυμός ροδάκινο instead of χυμός ροδάκινου, χυμός κεράσι instead of χυμός κερασιού e.t.c
Word order question. I thought that in Greek the adjective would precede the noun ... this phrase seems to follow the portuguese(! :) ) order of adjective following noun? Or does this only appear that way because it is an abbreviation as explained in @Theo_Matrakas' comment