FretBoards require X11 color names

[Aaron Hill]

Possibly silly question, but why are colors forced to be X11 in fret 
diagrams?

When you recolor a NoteHead for instance, you just use the literal color 
value:

%%%%
\version "2.20.0"
{
  \tweak color #'red             b'4 % symbol is invalid, defaults to 
black
  \tweak color #red              b'4 % color value stored in variable
  \tweak color #'(0.9 0.5 0.2)   b'4 % custom color as an RGB list
  \tweak color #(x11-color 'tan) b'4 % looking up X11 color
}
%%%%

You cannot do this with fret diagrams, as the internal logic assumes 
colors are X11 names only.

Not being able to specify a literal color feels inconsistent and 
arbitrarily restrictive.


-- Aaron Hill