"My car is white and her car is red."
Translation:Mi coche es blanco y el coche de ella es rojo.
I'm new to Spanish but...."ella coche es roja" would be wrong for a couple of reasons. First the car is what is red not the female owner. Car is masculine so you need rojo. Part of this lesson is to teach you how to use ownership words such as "de" and "del". She owns the car so duolingo wants you to start using these words. Personally, I do not know if it is ever proper to use the structure you used.