In fact the grammatically correct form of the adjective, which is NOT a foreign word, is "ο λαχανής - η λαχανιά - το λαχανί" so what you said is correct, but unfortunately many people in everyday speech erroneously say "λαχανί", "πορτοκαλί" etc, for all genders, numbers and cases as if it is non declensible.
I agree with those who assert that this adj is not foreign to Gk, because it is very close to the ancient Gk noun λάχανον, "vegetable." If someone wants to argue it is foreign to Gk, the burden is on that person to show where it comes from specifically, which language. It's a Gk lexeme for sure.
Since colors in different languages don't always refer to the same thing, it would be great to have a color chart for the different colors. Like where is the dividing line between Λαχανί and πράσινος or μπλε and γαλανό, or these for red I found online: έρυθρος (blood), πυρρός (fire), or just "plain" κοκκινο, etc.?
I just did a search and found this cool chart, which shows the colors as Greek men see them, and as women see them: http://www.iefimerida.gr/sites/default/files/xrwmata-ginaikes-andres.jpg from an article I can't read (yet?) http://www.iefimerida.gr/news/100400/Πώς-βλέπουν-τα-χρώματα-οι-γυναίκες-και-πώς-οι-άνδρες-εικόνες. This last link you have to select to get all of it. Interestingly, it doesn't include λαχανί, γαλανό, έρυθρος, or πυρρός, so I guess the names change as the whim of the style/color industry.
Anyway, it would be a cool thing if DuoLingo authors collected color names with color charts in the different languages to see where the differences lie!
there is also σάπιο μήλο (rotten apple), ποντικί (<ποντίκι=mouse), σταχτί (<στάχτη=ash), καρπουζί (<καρπούζι=watermelon), λαδί (<λάδι=oil), καναρινί (<καναρίνι=canary), θαλασσί (<θάλασσα=sea), βυσσινί (<βύσσινο=sour cherry), κεραμιδί (<κεραμίδι=roof tile), ασημί (<ασήμι=silver). But I don't think there is a clear dividing line between them. At least for me.
That's highly unlikely. All translations include the word κάλτσες, and the tiles are always picked from the best translation. Please, make sure that you didn't miss the word, next time you come across this sentence. If it still doesn't show up, please send a screenshot form your PC or mobile device attached to a comment so we can confirm your sayings. ^.^
"Ανοιχτό πράσινο" is more usual than "λαχανί" in many contexts.
Λαχανί is light and bright or fluorescent green. It's never ever dark green. (Yes, λάχανο is cabbage.)
Dark green is "σκούρο πράσινο". It can also have different shades.. for example a shade of σμαραγδί (from σμαράγδι precious stone) or dark brown-green λαδί (from oil).