A color mask only depends on pixel colors and can be configured to perform either color keying or color selection based on the RGBA channels.
The color selector function can be changed to many different function types such as
Define a function using keyframes
or Color Tone Balance:
You can also do some chroma key using presets, or by turning the color selector type parameter from "channel" to "key" in order to select a specific color for this mask.
In this case, feather" and tolerance are the only parameters you can act on.
-tolerance : distance of the color range from the one you picked up.
-feather : Smoothness of this range.
This modifier has a 3D equivalent :
Vertices are selected depending on their color
here is a small tutorial to do chroma key with this modifier: