They Magic Marker them black and just tape them on. It's good enough for an experiment though not for anything permanent. Trap is a vertical game. Skeet, particularly shootoff doubles where the game is really played these days, is mostly a horizontal game. Also, trap shooters usually use two different guns for three quite different games, so doing what ever is necessary to get a similar sight picture, while dealing with different points of impact, is at a premium.
Many skeet shooters use only one gun and the game is always the same.
If they do use a second gun, it will be an auto 12. For a skeet shooter it might be a little easier to get used to a certain sight picture that your gun demands, rather than modifying your gun for a certain sight picture that you demand. Obviously, if the differences in what you want and what you have are extreme, then you have to do something. But usually for skeet, you have some flexibility. At lot of it is just what you get used to.