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.