Better colour planning

[jcrippen]

Currently the prefix colours are sort of arbitrary. Before we get locked in to a particular pattern, it would be useful to figure out some ideal sets of colours to use.

Issues to consider:

• background colours: Currently the assumption is a white background. But what about e.g. a black background, or even a Solarized (https://ethanschoonover.com/solarized/) dark or light background?
• print versus screen: The current colours are RGB, designed to work on screen. Print colours are generally CMYK, with spot colours also available. Good RGB screen colours may look like crap in CMYK print, and even good CMYK colours might look like crap when blended by a cheap printer.
• adjacency and contrast: Some colours are hard to distinguish when adjacent. We should try to ensure that colours are maximally contrastive for adjacent characters in all the possible combinations.
• relative contrast: The current colours have been eyeballed to not contrast too much against each other in terms of their perceived luminance, but this is all guesswork. It would be very useful to look at some perceived luminance models to more reliably select various colours in opposition to each other.