Changing Primaries

I've added the ability to change the primaries that are displayed on the mixing palette. This doesn't change the results of colour mixing, but is more a usability feature to allow a user to customize the "quickly available" colours for use in mixing, ie, the colours they consider to be their primaries.

The image shows the mixing area with a different set of primaries than those used in other screen shots.

(For comparison, here are the colours used as primaries in the other screen shots:)