A display driver is responsible for translating logical colors (RGB values) into physical colors that are appropriate to the display hardware. Similarly, it must translate physical colors to logical colors. GDI calls the ColorInfo function whenever it needs a translated color.
If the display device provides a color palette, the driver converts colors to palette indexes. To indicate a palette index, the driver always sets the high byte of the index to 0xFF.
To indicate an RGB color value, the display driver sets the high byte to zero.