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.