When there is a wide range of light and dark in a color composition, that composition is in a "major key" of value. (If you have a hard time perceiving value, imagine your composition broadcast on a black-and-white tv....what you see are the different values of the colors... each color corresponds to a particular shade of gray, whether it contains gray or not. Snow pictures are ideal for noticing value because we naturally perceive them as blacks, whites, and grays, even when other colors are present.)
A narrow range of light colors or of dark results in a "minor key" of value.
For yarns and rovings, I tend to prefer a minor key. If I am spinning along a roving and come to a spot that is much lighter or darker than the rest, it jars me somehow.
And yet, mixing lights and darks in a knitted object can be quite pleasing to me. I simply prefer using
light and shadow (i.e., a textured stitch pattern)
or several different yarns to achieve that end.
What is your preference for dyeing? For spinning? For other fiber crafts? Major key or minor key? Or does it depend on the project?