Transparent denotes the alpha channel is has a value of zero. Rgb 0,0,0 is black 255,255,255 is white thus 255,255,255,0 would be transparent white 0,0,0,255 opaque black and 0,0,0,0 transparent black.
These values are of importance when blending colors under Non Premultiplyed alpha. Earlier versions of xna defaulted to that.