3F00,04,08,0C,10,14,18,1C should all be considered transparent. Writing to 3F00/10 sets the background color. Writing to 04,08,0C,14,18,1C has no effect.

For emulation purposes, it's often suggested that 3F00/10 writes are mirrored to 04,08,0C,14,18,1C. This can produce the same effect as above, but technically it's incorrect.