The orange dutch colour. - was marked wrong. Why?
Orange is a Dutch colour - sounds, as if the Dutch developed this colour first. It is not even in their flag nowadays. What is meant with this sentence?
They are looking for sentences that use colours and recycle some of the language we have learnt without worrying about a lot of meaning. It's still the colour for lots of their sports teams, though, and apparently the reason most carrots have been bred to be orange!
I would suggest that similar to what Duo Tips taught in an earlier lesson concerning general statements about animals, this general statement concerning a color equally requires a definite article. If there is no definite article the indefinite article is implied.