when you are quilting an edge to edge design, you probably need to stick to one color thread. Variagated may work well--but do remember that most varigations are not random, so you may end up with a secondary design via the quilting. If you are going to switch thread color you will probably want to do more custom work in each color fabric. another idea, one that I use a lot, is a gold or a silver color (I don't use metallics, but instead a poly thread in those colors) and it works pretty darn well.