A "mask" is treated as having an opacity related to its color. The "darker" the color, the less opaque it is. The extreme ranges of a mask are WHITE (completely opaque, hexadecimal FFFFFF) to BLACK (completely transparent, hex 000000), when using a gray scale, non-inverted mask. Note that the hexadecimal number is a number system that counts from 0 to F (decimal 0 to 15). It's a quick way to represent a number based on 0's and 1. So 0 is off and 1 is on. Counting 0 to 8: 0000, 0001, 0010, 0011, 0100, 0101, 0110, 0111, etc until you get to 1111. So, effectively, you have completely black at 000, completely white at 1111, and some shade of gray for all numbers in between.
So, any color can act as a mask and the mask's color acts as some shade of translucency, depending that color. The maximum effect for the mask is seen at the color contrast extremes (or, white to black). This masks summary should help:
http://www.fenimorephotovideos.com/Pros ... sions2.pdfDale