Validate Colour setting

[DavidGarfinkle]

Music21 offers two major outputting formats: xml and lilypond. However, they differ in their support for score colouring.

RGB and Hexadecimal
XML supports RGB triplets (though I haven't gotten them to work yet through music21) and hexadecimal colour specification (this works)
Lilypond supports RGB (through an internal scheme function which I also haven't gotten to work through music21). As far as I know Lilypond doesn't support hexadecimal colour specification

http://lilypond.org/doc/v2.18/Documentation/notation/inside-the-staff#coloring-objects
https://usermanuals.musicxml.com/MusicXML/Content/ST-MusicXML-color.htm

Keywords like 'red'
Both XML and Lilypond support basic colours keywords, like 'red', 'orange', 'green', etc...
Lilypond has a full list of their supported colour keywords here: http://lilypond.org/doc/v2.18/Documentation/notation/list-of-colors
I can't find a similar list for MusicXML colour keywords.

So for now, we'll work with basic colour keywords that work in both xml and lilypond, but it would be nice to support at least one type of exact colouring schema.