A mother color is a color which is used in every mixed color in a particular painting. You can select whatever color you wish as a mother color. It can be a neutral gray or brown mixed from the leftover colors on your palette, or it can be a straight-from-the-tube color. The logic behind doing this is that it harmonizes all the colors in an individual painting, creating a unity in the color and composition.
The mother color can either be used as the starting point for the colors you create by (a) mixing another color into some of your mother color,or (b) by mixing a little of your mother color into every color, or (c) by under toning the ground with a single color. And finally you can use a mother color to harmonize a painting by finishing the painting by glazing over tall the other colors with a single transparent mother color. I've tried all of these methods but my favorite methods with oils are (b) and (c).
When I am painting a scene that is dominated by blues and greens I often paint tone the entire canvas first with a medium shade warm hue like cadmium red light or cadmium orange. I try to choose a shade that is a natural complement to a hue right in the middle of the range of blues and greens I will be using. The warm hue will show through in areas where there is no paint adding sparkle, and it will warm other cool transparent colors like ultra marine blue.
When I am painting scenes that require an overall warm palette, I often use a blue hue as my mother color. Blue by its nature ( even a warm blue) is cooler than most of of the hues I uses in my paintings, so blue adds a nice touch of contrast.
Which blue you ask? My four main choices are: Cerulean, Cobalt, Phithalo, and Ultra Marine Blue. The one I use depends upon the painting. I let the actual scene tell me which one to choose.
A mother color is that it can be used as a dominant color (or color theme) within a painting, or it can be used less prominently. One problem with using a mother color too strongly is that the colors are too similar (in tone and hue).