The easiest way is, of course, to find a fan-created download that overrides the default skin tones and put them in your Downloads folder.
If you want to make your own replacement skin tones, that's a little trickier. You would need to:
- Go into C:\Program Files\EA GAMES\The Sims 2\TSData\Res\Sims3D and, with SimPE, find the files that contain the default skin tones.
- With SimPE, extract the skin tones into new files.
- With SimPE, edit the textures in the new files. (There are about 32 textures for each skin tone.)
- Put your new version of the skin tones into your Downloads folder.
This will create a set of files with the same internal ID as the ones that are packaged with your game. Since the game loads the Downloads folder last, you new default skin tones will override the Maxis versions.