For when these are used, see Component:PBS_ColorMaskSpecular and Component:PBS_ColorMaskMetallic
Color maps are defined by 4 channels specifying 4 different colors to blend between. RGBA are the channels; which represent red, green, and blue respectively, with A being alpha or transparency.
When these channels are not exclusive to each other on a pixel (so there is a mix) the 4 values for RGBA are used to mix the 4 colors associated with that value.
Despite popular belief, Black is not it's own channel, because it's just Alpha with the absence of RGBA, which creates 100% of the 4th specified color on the material.
if it's Black and transparent, then black as a color will be used as a default.