Since oil paint stays wet for so long, many artists use a "wet on wet" paint application instead of waiting for the paint to dry before applying new layers. This means that when you're painting a shadow on the canvas, the color of the shadow will mix subtly with the color of the object you're shading. This technique is useful because the two colors will blend and seem natural together on the canvas.
Often artists disagree when it comes to the differences between scumbling and glazing. Many people describe "glazing" as the application of a thin layer of transparent paint over a thicker layer of paint, left visible beneath. Scumbling is the application of a layer of paint with a dry brush over a dry, textured layer of paint, so that the wet paint catches on the dry layer, creating an uneven top layer of paint. A scumbled layer of paint allows some of the layer underneath to show through. Glazing and scumbling are both methods for applying a thin layer of paint, sometimes a shadow.
If an artist leaves a canvas out in the open for days and then comes back to it, the paint will be superficially dry. Trying to apply a layer of shadow on a dry painting will often require an artist to first paint a fresh layer of paint on a previous layer, matching the new color to the old color, and then blending the additional color for the shadows with the new layer of wet paint.
The first instinct most people have is to shade with black paint, because shadows are dark. The problem is that black, when mixed with most colors on the canvas, creates a murky, muddy mess. Black makes subjects look dirty or discolored. Instead of using black, colors like blue, deep red, green, brown and purple may be used. Many artists will blend the color of whatever object is falling into the shadow with the complement of that color. Complementary color pairs are those colors that appear opposite each other on the color wheel.