Sorry for my late answer - had a lot of work.
My proposal is not only for select color of a lamp (colorpicker), but more for dynamic color change with combinators.
RGB model is more simple to me, for example I can easily build combinators to show battery level (from green to red).
I can likely solve ...

TL;DR
New "RGB" signal works as normal color signal, but when used in device with "Use Color" checkbox (e.g. lamp) device will use it value (with clamp [0, 0xFFFFFF]) to set custom rbg color.

What ?
Add new kind of color signal which can be used as RGB color value for specific devices (with ...