Totally legit sentence - unripe oranges are green I think. And also a great opportunity to make a point about the difference between the two words in Hebrew :)
The thing is, it's not funny in Hebrew, in Hebrew this is a completely normal sentence: it's a simple way of saying you want ripe oranges.
Actually green oranges can be perfectly ripe, like those coming from Brazil and used for juices. the orange oranges are those which went through a cold winter, IMHO.
The first language I've seen that didn't derive both words from the same root :D
That's because תפוז is actually an abbreviation for תפוח זהב, i.e. "apple of gold"
This is the first language I have seen where orange (the color) and orange (the fruit) are totally different words.
III כתם kétem means "stain", and from there you have words like מוכתם, mukhtám, "stained". It's probably the same k-t-m root as for כתום but I'm not 100% sure.
It's interesting how they are 2 different words for the color orange and the fruit. To answer your question, Shira, duolingo automatically hides you if you have too many dislikes.