Swatches are a simple ASCII format, which is line-separated colors, in format R G B A.
Not really sure what else I'd need to say about that. To add black, you would put in:

0 0 0 255

White would be:

255 255 255 255

MCSkin3D comes with a few swatches for samples, such as Base (primary/secondary to black fade), Net Colors which is all colors that Windows defines, System Colors which is the base system colors, as well as Skin Colors and General Colors, graciously donated by White Dragon, which were made for their obvious purpose.