GDP: Colors in RGB?

[Trevor Daniels]

I'm helping to write the GDP manuals, and am puzzled by an
omission in the description of setting the color property of
grobs.  The Internals Reference (grob-interface) says the
color should be a list, and supplying a list of RGB values
like

    \once \override Voice.Slur #'color = #'(1.0 0.0 0.0)

certainly turns the slur red.  But this way of overriding
the color is not mentioned in any of the user manuals, at
least not that I can find.  Is there a reason for this
omission, or may I describe this method (as well as the
standard ones) in the new GDP manuals?

Trevor D