Colour masks look and act very much like Luminosity Masks, but are actually masks based on colours in our images. Where a Luminosity Masks makes selections in our images based on brightness values, colour masks make selections based on the amount of each of the colours in a pixel (Red, Green, Blue, Cyan, Magenta, Yellow and Orange).
The light areas of the selection using colour masks is telling that those areas contain a lot of the colour in the chosen mask, the darker areas represent low values of that colour, therefore it looks a lot like a Luminosity Mask, and allows us another method of making selections if we are unable to obtain the desired selection with a Luminosity Mask.
Intuitively choosing a colour mask, you will think that your mask will show white in the area that are visually that or close to that colour. But in actual fact every pixel in our images contains a little amount of each colour, or almost every pixel. You may also wonder why the white areas of the image show up as white in the mask, and the reason for this is in order to create white as a colour you need 100% saturation of Red, Green and Blue.
I encourage you to look at these masks when you are having difficulty creating the selection you want in Luminosity, the perfect mask may be sitting there waiting.
The video below will show how to create colour masks in more detail.