Gamut masking is a technique to limit the range of colors available for a painting, in doing so, it's easier to achieve an harmonious painting or illustration. The idea comes from James Gurney who explains it really well in his fantastic book Color and Light.
The gist of it is that you place a shape, usually a triangle, diamond or rectangle, over the color wheel so that it masks the colors you don't want to use and allows you to work with the ones you want. Moving this mask around or transforming it in any way allows you to create and discover new color schemes.
These gamut masks, or wheel masks as they're called in the app, feel sufficient as a color scheme. The gamut they define, that is, the range of possible colors available for a given work, contains subjective primary and neutral colors that make a complete and harmonic color scheme.
They're also very useful for creating color scripts in sequential art forms, such as comics or storyboards, allowing you to come out with diferent color schemes that will make the transition from scene to scene much clearer.
For example, I've painted the following drawing using two different wheel masks as reference:
For the first one I used a complementary color scheme with a saturated blue and yellow while every other hue is quite unsaturated. In this mask, there are four subjective primary colors, the blue and yellow mentioned before, and a desaturated red as well as a desaturated yellow. The neutral color is just grey because the mask is centered on the wheel.
The complementary colors for this gamut mask are all unsaturated. As in all color wheels they're the colors found between the primary colors, in this case they're orange, pale green, cyan and purple.
On the other hand, for the second one I used a triadic color scheme with saturated orange, turquoise and purple acting as primary colors. The complementary colors are less saturated red, green and blue, and the neutral color is also grey because this mask is centered on the color wheel too.
Although I didn't particularly try to use similar colors for the salamander-like beast on either version, I actually used similar hues to paint the background. It's easy to compare them and notice that the first version is way less saturated than the second one. Yet, there's no problem in separating the greens from the cyans in either of those images because the maximum saturation of each hue is kept relative to the saturation of it's neighbors in each mask.