Toning down a color can prove to be a real problem if you do not know how to make it work and not turn to mud. If you add black it will turn dull and drab and here is where the truth lies. When a color is too intense add the opposite color, such as: to tone down yellow add violet, for red add green.

Complementary colors and also intensify them, take two colors, blue and orange, place them side by side, now the blue is brighter and the orange looks oranger. This is how the impressionists made use of color. Lemon yellow looks brighter with violet, and red look redder with green next to it. If you add red to the green foliage it will appear duller, thus making the other green stand out.

The color of the shadow depends on the color of the light. Lets take an apple, the shadow could be green. An orange could use the shadow color of blue. Both are cool colors and shadows reflect cool colors. Violet could even be considered in the shadows.