"Welche Farbe magst du am liebsten?"
Here's a breakdown:
* Welche - Which
* Farbe - Color
* magst - (you) like
* du - you
* am liebsten - most of all (favorite)
You can also say:
* Was ist deine Lieblingsfarbe? (What is your favorite color?)
* Welche Farbe hast du am liebsten? (Which color do you like best?)
All these options are correct and will be understood by native speakers.