Formal:
* Quelle est votre couleur préférée ? (pronounced: kel eh votr kloo-rur prey-feh-ray?)
Informal:
* Quelle est ta couleur préférée ? (pronounced: kel eh ta kloo-rur prey-feh-ray?)
The difference between the two is the use of "vous" (formal "you") versus "tu" (informal "you"). You would use "vous" when addressing someone you don't know well or are being respectful to, and "tu" when addressing someone you know well and are comfortable with.