You want contrast, but it does not have to be black or white. I have a quilt right now that is various shades of rust/reds with a offwhite/ecru as the contrast. And even the contrast depends on what you want to achieve with your design. My quilt has a star in the center, and is made up of mostly half square triangles...I want a lot of contrast.
If I were doing a quilt that was a landscape, or art quilt depending on various shades of color (think going from sky to water to grass)...then I'd want a very minute change of color, and not a lot of contrast.
There are as many ways to make a quilt, as there are quilter's out there...do what pleases you.