You can do RGB on hair by editing the XML files. That said, IIRC it doesn't actually *work* unless you've got the hair styler installed (otherwise it falls back to the style's default colour?)
For example:
<Hair>(...style...)</Hair>
<HairColor>123,123,123</HairColor>
(PPP actually generates colours like that if you use the "RGB xxx" colour options)
The colour is applied over the "white" hair texture that's available via the hair styler, and it looks a bit "flat" to my eye, but tastes differ.