I have a 160x128 colour OLED and am seeing odd results when using xor mode.

OK, clear screen to black (0) and then do -

"SF",123,"SC",1,"ETP",0,50,"DM^","TT0",13

The end result is a white square. I expected my character to appear in white on a black background.

If we try again but draw a blue (0b111) rectangle to clear the screen, the end result is a yellow character on a white background.

This is a rather strange xor function!  Why is anything happening to pixels that aren't part of the character that I'm drawing?

 In order to compatible with mono module, the color 0 was assigned to black, and color 1 was assigned to white.

If you want draw white letter on black back groud, use mode "C"-copy.