Agreed. However, I would probably say "the yellow one" in most cases. If there were a bunch of tubes of paint, then I might say only, "Are you taking the yellow." Also, I never know if Duo is giving me sentence fragments or not. In some of the courses, sentences can sometimes be incomplete.
But the Dutch sentence is not about willen (to want) but about nemen (to take). I know these can be roughly the same, say in a restaurant setting. But this doesn't have to be the case, e.g. you own two cars and you're asking your partner which car he/she will take today, I don't see anything wrong with using to take in that kind of context.