Gamut Masking is an easy way to create coherent color schemes. This selection of a set of colors is done before starting the painting process. It saves the artist from a lot of color confusion later and automatically gives a unified look to the painting.
Gamut masking is similar to a limited palette but there is an additional dimension to it which makes it more efficient. With a limited palette, you almost always start with primary colors and colors with the highest possible saturation (as they are taken directly from the tube) whereas, in gamut masking, you generally start with lower saturation colors or even with secondary colors instead of primary ones.
Notice that I used the word "generally". That's because technically you could start with primary colors with full saturation and still call it a gamut mask, but that's not taking advantage of the gamut mask method in its true sense.
An example of a painting in different color schemes
See the photo of a painting I did today. Next to it are the images of the same painting shown in different colors. See the change in the mood it creates. High contrast and vibrant colors create a happy and cheerful mood whereas low contrast and low saturation colors produce a cooler and melancholic look.
Original painting, same painting with vibrant blue and the same painting with vibrant orange
Example of Gamut masks
Here is an example of how you could use a gamut mask for selecting colors for your painting.
No mask, Mask 1, Mask 2, and Mask 3
Take a color wheel and overlap some shape on top of that. Only those colors that are inside the shape should be used for painting. Try different shapes to see which colors fall inside it. Changing the size and/or shape of the mask will change the colors you can use.
As an exercise, paint the same painting 3 times using different color schemes (given by 3 different gamut masks) you chose and see what feelings each painting gives rise to.
Hope this gives you a new way to select colors for your next painting. Use this method and let me know what you learn!