CTexmap is not just AO pass maker, it's render element to make arbitrary passes out of shading networks.
If you create for example AO pass, and don't want to be cutting it out with the mask, just apply overall to the scene, you will probably want your BG color to be white, so if you multiply it over beauty, your background won't turn black.
At the same time, if you want for example mask of certain bitmap features on bottle etiquette, where white is contents of the Mask, and black is empty area, then you will probably want BG to be black as well.
So since you already need two different colors, then it makes most sense to just change ignore background checkbox to override background color checkbox, and stick a color swatch in there. That way, you can set it either black, or white, or any other color. Since it's for arbitrary passes, there may be some rare cases where you may want some other color than black or white.
Pokoy has requested something like this some time ago for Cmasking_ID pass, where I wasn't sure it would pay off, but I stressed several times it would indeed be useful for CTexmap, and since you are another one who requested it, it should be easy to push it through.
