Avoid using black in your darkest darks, as well as to create gray shadows for white. Whether you are painting with oils or wool, your work will more have life, dimension, and interest without it.
I like to create black in oils by mixing or layering transparent dark colors - Veridian Green, Alizarin Crimson, and Ultramarine Blue. You can slant your black in any of those color directions by adding more of one color, or by making it the top layer. You can also lean toward brown by adding a touch of yellow.
|A white shaggy dog on black (not really) back ground - All of his shading is variations of purple, green, and cream.
|A black and white cow... or is it?
|Unfortunately this painting created a lot of reflection in my photograph of it but it is full of fun colors.
Wool is a little different because we can not blend colors with a palette knife. To some degree we can blend and layer but it requires a slightly different aproach. With wool, I do use black as an under-layer for dark areas of my felted painting as well as very black subject matter. The underlayer helps to establish that the area will be dark. As a top layer, black makes my darkest darks and I can hightlight with teal or purple from there.
I do not have very many wool paintings with which to illustrate these ideas, but I sure look forward to making them!
|The white underbelly of this hummingbird is in shadow and created with soft lilac and tans.
Let your colors be less literal. Let the paint or wool speak its language.
Of course there are always exceptions to the rules!