Is "màu" obligatory in this sentence because otherwise it would mean the dishes full of oranges?
The problem with that is the fact that the English phrase orange dishes can also mean dishes of oranges, so in fact Chúng tôi thấy những cái đĩa cam is the best translation because it reflects this ambiguity most accurately.
why is "xem" marked as incorrect?
Xem = watch, you are doing the action on purpose.
Thấy = see, the object just accidently appears, you don't expect to see it the first place.
Xem is usually used for something that has content, like a movie or a document or a picture.
I use thấy as shown and it corrects me to same. I will try copy and paste the "correction" and see if it will let me pass.
It let me pass. Now I came through here again and same thing happened. Back to the ol' copy and paste.
read the sentences as the print size changed from normal to tiny. I not officially hate doing these sentences. Too bad I am learning Vietnamese or I would quit.
Is cái really necessary in this sentence? It doesn't seem like it'd be necessary in this case.
It is necessary. Between (những, các, vài) or numbers (một, hai, ba,...) and noun, you usually need a classifier.