Mud happens. But why?  That question hangs over every artist's palette during   mixing trials. If a mixture proves undesirable, all too often another color is added, then   another, and the mixture turns muddier with each. Colors can corrupt each other,   especially if their hues conflict (not color wheel close). At least some brilliance is lost in   any mixture. Each color relfects a specific portion of the light spectrum and absorbs   (blocks) the rest. In mixing, one color blocks some of another color's reflections, i.e. less   light, less color. The more hue-diferent the colors, the duller the results. Mixing in more   colors makes for a "muddy" dirty appearance due to increased light reflection blockage.   Keep mixtures simple, just two colors if possible. Consider "close enough" straight-from- the-tube color whenever you can.

WHITE is pure and innocent. Full spectrum sun glare is the whitest and brightest that white can be. (It makes us squint.) All others are less white, reflecting nearby colors and imparting the psychology of those colors. Bluish, yellowish, greenish, pinkish, etc. versions of white communicate color associations with more subtlety than full strength, unmixed colors.

Read more about color associations in the Psycolorgy: Seeing What You Think section of Caroline's book POWERCOLOR - Master Color Concepts for All Media.