While it may seem logical that to lighten a color you add white to it and that to darken it you add black, this is an oversimplification. White reduces brightness so although it makes a color lighter, it removes its vibrancy. Black doesn't so much add darkness as create murkiness (though there are instances in which black is uniquely useful, such as the range of greens it can produce when mixed with yellow!).
Why Can't I Add White to Lighten a Colour?
Adding white to a color produces a tint of that color, makes a transparent color (such as ultramarine) opaque, and cools the color. This is most noticeable with red, which changes from a warm red into a cool pink. You can add white to lighten a color, but because this removes the vibrancy of a color you'll end up with a washed-out picture if you use white to lighten all you colors.
Rather develop your color mixing skills to produce hues of varying intensity. For example, to lighten a red, add some yellow instead than white. Watercolor paints are, of course, transparent, so to lighten you simply add more water to paint to let the white of the paper shine through.
Why Can't I Add Black to Darken a Color?
Black tends to dirty colors rather than simply darken them. Of the most common blacks, Mars black is the blackest and is very opaque, ivory black has a brown undertone, and lamp black a blue undertone.
