questions - "Which" or "what" for accepting multiple choices from a list

I know that, in the context of radio buttons (the options are limited and they choose one), I should use which. For example, Which is your favorite color?

( ) blue
( ) green
( ) red

I also suspect that for an unlimited text box, the correct option is what. For example, What is your favorite color? ________________

My instinct tells me that the same is true for checkboxes. For example, What colors do you prefer?

[ ] blue
[ ] green
[ ] red

When they can select multiple answers, I should use What colors do you prefer?, not Which colors do you prefer?, right?