Following up on #516.

With the new execute API the ColorScheme class will start to play a more central role.

I’m considering making the ColorScheme class immutable. This will be a breaking API change. Should it be deprecated first, or not changed at all, or is the upcoming 4.0 release a good time to make breaking changes?

Feedback very welcome!

(cc @bobtiernay-okta @bbottema )