Reference: pmRGBSplit

Essentially, the pmRGBSplit function extracts the red, green, and blue components of a pixmap.

pmRGBSplit(pm, r, g, b), where pm is a pixmap, creates three new pixmaps, named r, g, and, b (which have to be strings representing valid identifiers), where r, g, and b are copies of pm, but

 * r has all green and blue components equal to zero,
 * g has all red and blue components equal to zero, and
 * b has all red and green components equal to zero.