ColorCodes: simplify and comment a bit.

- Simplify logic and increase explanation to make it easier
  for the casual reader.
- Remove the undocumented, untested and not-clearly-working
  "percentage" support.
- Replace `eval` with `try`, as is my wont.

Addresses #620.
Everything is in place for things to happen at "any moment" on the CSS front.  The back of the house action is fairly fast and furious these days with some third party product integrations taking center stage.  I expect to get the CSS portion done in the next couple of days and then start to do the deep think on the templates.  The infrastructure bits are more or less in place there, but we want to get it right out of the gate.