This is an old page. The information is much better elaborated in my book Color Movement Theory
Consider one primary color on the color wheel. See how one color leads into another? Start with a primary Yellow, but let it slide a bit towards Red. Now we have a color moving towards Orange, but not Orange. It might be a tertiary, or it might not reach as far as a tertiary. What is important to consider at this point is that sense of movement.
This is a sense of movement of one color transforming into another. This is important, especially as we consider color mixing.
We are also going to look at the movement between colors.