Quite simply... a screen. Red wine often will have sediment from the fermentation process, particularly older wines. The screen will help to catch that sediment so it does not go into your glass, and eventually into your mouth. White wines typically do not have sediment as the winemaking process differs a bit from that of red wine. Plus of course the white aerators are generally white in color which you may find more attractive. It is also nice to have both as the red wine can leave some residue which you don't want to mix with your white wines. But the way they work and how they aerate is really the same regardless if you are using a white or red aerator.