Just have the sims visit a community lot and purchase new clothes. Then when they return to the neighbourhood, make sure they have a dresser in their home, and click on it to Plan......(eg everyday or swimwear or whatever) to change their clothing.
Ahhh ok, sorry. You can't change the skin tones in-game after the sim has been created. However, using a mirror, you can change their make-up and Costume make-up which may alter the facial colour tone. This is only superficial, as genetically the sim will retain the skin tone selected in Create-A-Sim.
Actually there is one option to change the sims skins after they are created. But it will involve changing everyone's skin tone who has that same skin tone. If you go over to MTS2 and search for default skin under the downloads section you can download those to replace the current skin tones you have. Basically it just overwrites the maxis defaults. Or just do a google search and you might come up with even more. I believe you can also create your own defaults by using SimPE, but I'm not positive on how to do that myself.
