Question

I saw in some image processing software, we can specify “color key color”, usally it is 255,0, 255 in RGB. what does it mean?

Answer 1

color key color means the ‘invisible’ color. Or, by other words, the transparent one, like an alpha channel. Directly related to the Chroma key http://en.wikipedia.org/wiki/Chroma_key
Also read this one http://www.saltw.net/index.php?topic=82117.0 part of the text named Using Color Keying on 2D Sprites

What does this mean to you in this stage of computing? If you look
back at the last image of where we left off you would see there are
big white boxes that don’t blend to the background. What we want from
the sprite is just the plane or the helishark. This problem existed
back then so one color in that 256 color pallete was called a Color
Key. This key when read by the computer would be understood to ‘don’t
draw this pixel, it is invisible.’ It’s still used today as it is a
very simple concept: If a pixel is this color, don’t draw it!