NOTE** This is an older method that I have stopped using in favor of the Skinner Blend technique. This polymer clay method translates easily to clay by using a slab roller instead of a pasta machine. You can even use a rolling pin.
Look for my article on this technique in the January 2013 issue of Pottery Making Illustrated.
Edition #35 of Ceramics Art & PerceptionTechnical
To create a third new color, I mix two existing colors together. I weigh each color then shape it into a sloped triangle shape. I almost always add i part of white clay for each part of colored clay since I mix very saturated colors.
Flip so the thickest end of one color matches the thin end of the other.
I slice the resulting loaf into five or six numbered pieces.
I knead each slice separately and re-number it.
For more options and information on coloring clay, look at the WORKSHOP pages of my site to find a class near you. If there isn't one, consider recommending my workshops to an Arts Center in your area.workshop area