Source and destination color keying for blits and overlays are supported by DirectDraw. For both of these types of color keying, a color key or a color range may be supplied.
Source color keying specifies a color or color range that, in the case of blitting, will not be copied, or, in the case of overlays, not be visible on the destination. Destination color keying specifies a color or color range that, in the case of blitting, will be replaced or, in the case of overlays, be covered up on the destination. The source color key specifies what can and can't be read from the source surface. The destination color key specifies what can and can't be written onto, or covered up, on the destination surface. If a destination surface has a color key, only the pixels that match the color key will be changed, or covered up, on the destination surface.
Some hardware will only support color ranges for YUV pixel data. This is because YUV data is usually video, and, due to quantitization errors during conversion, the transparent background is not a single color.
Content should be authored to a single transparent color whenever possible, regardless of pixel format.