I use different colors both for piecing and for quilting. How many different colors I use in a particular quilt depends on the effect I want to achieve-- for example, if I want the quilting to be mainly a texture I'll use threads that blend in with the fabrics being used, so I might need to use multiple colors to match fabrics if the quilt is scrappy. Or, I might want the quilting to stand out, so I might use multiple colors chosen to show up against the fabrics.
Rob