I believe supported colours are white (ffffff), cyan (00ffff), magenta (ff00ff), yelllow (ffff00), blue (0000ff), green (00ff00), red (ff0000), and black (000000). This corresponds to each red, green, or blue subpixel being either on or off, the only supported modes when the screen is in low-power mode. White is the smoothest colour, since all three subpixels are switched on. Choosing cyan, magenta, or yellow will result in slightlier 'jaggier' outlines in your chosen picture, while the roughest looking will be an image in any of the three primary colours, red, green, or blue, as only a third of the pixels will be used to draw the image. At least that's how I understand how things work on the pentile screen.