Color addition select.
Format: ccmm--sd.
cc = clip colors to black before math
mm = color math prevention
s = add subscreen (replaces fixed color)
d = enables direct color mode for 8bpp graphics (modes 3, 4, and 7)
For cc and mm, the values they are set to determine when they apply: 00 = never, 01 = outside color window only, 10 = inside color window only, 11 = always.