You can "extract" the color information from an existing part, although it's a bit laborious to develop a new swatch.
Have a look at the HELP file for "swatch" instructions:
Under Color Properties, define the color you want to add. Under Favorite, click Add Current Color to Swatch.
The steps above can be repeated during the initial swatch creation in order to populate it with multiple colors; however, I find (in 2006 SP4.0 at any rate) that, once the swatch is saved, there can be no new colors added or existing ones removed. (The .sldclr file seems to somehow become read only...)
I would suggest that you select a specific face or feature color and then go to the Color And Optics Property Manager to make manual notes of the R,G,B values and optical property slider bar settings. Make a list for all the "custom" colors you want to capture in a new swatch BEFORE you begin creating it.
Then with those numbers available you can enter the colors precisely rather than having to poke around in attempting to guess the properties.
Alternately, you could select a specific face or feature color to have it show as the "current" one as the Color And Optics Property Manager is opened. Then a new swatch could be saved with that custom color as the only one in the file. (I haven't found a way to get a second specific face or feature color (from the same open part) to show as "current" for adding to the swatch.)
This would be repeated for each and every custom color, but it really isn't efficient to have many swatches with only a single color captured.
Hope this helps. (I don't understand why colors can't be added or removed after the swatch is first created and saved, but at least a means exists to capture a custom group.)
Per O. Hoel