Allow vars in Colors setting

If in use with other frameworks e.g. ACSS, Core, Advanced Themer, you already have all your colours set up as variables.
It would be really handy to be able to reference these in the Colors panel (and for it to display them so you know it's got the right one) rather than specify them in hex twice, remember an additional place to change them etc.

(Side note - while allowing variables, a fringe use case would be to allow them to be used for the timings fields as well - if it's easy to do at the same time, it would be nice to be able to set these in code elsewhere. But that's not the main point of the feature request!)