Darkrp DermaSkin choices

Hello. I have made two skins for darkrp’s default dermaskin (just colored DarkRPSkin.png’s) they are green and black, well now i thought that it would be cool to make people a choice to choose if they want to use green or the black skin. Now i want to ask if someone would show me an example how i would make one, i’m not really sure how i should make it. Thanks.