This is one layout pattern to get a rainbow blend. Click on the picture
to go to a page with only the template on it. I've made the colors pastels
to save you some ink. ;-) Pastel rainbows are pretty, too. Try one! The
template for the two-color simple blend is here.
Can you see how, after you fold this sheet on the line that runs across the center, the clay between the green lines will have it's own "zone" that the adjacent colors won't encroach upon? You can make these areas of "pure" color wider by making the diagonals steeper, and you can eliminate the "pure" colors that you start with by making the diagonal cuts shallower.