The appropriateness of a response derives from the context in which it is asked. If the context is a math class, then appeal to the formal definition of a curve would be appropriate and appeal to color would not be. If you were at a paint store, the opposite would be the case. Answering with a color answer in a math class would be literally true but also overly literal. Nevertheless, it is not totally inappropriate in a math class, since precision and explicitness are desiderata in mathematics, and asking "What is the difference..." is really a poor question, since one can refine the question without giving away the answer. One can omit the color difference, which is just a distractor.
One can answer that one is blue and the other is orange, but I don't think one can formally answer that way: but the question does not require you to answer formally.