But apparently, LEDs require advanced processes unlikely to be available for makers, anytime in the near future.
However, Zinc sulfide phosphor mixed into epoxy can be used to make voltage activated luminescence. For some interesting guidance on "doping" for color, I present a 1953 patent:
> zinc" sulphide' activated by both copper ad manganese in accordance, with the present invention is strongly electroluminescent, the addition of. manganese to the copper-activated material resulting in a shift in the color of luminescence toward the red end of the spectrum. Thus, the materials of the invention, when excited by a fluctuating electric field, show colors of luminescence ranging from bluish-green through shades of bluish-white, pinkish-white and yellow to orange. [0]
Looks like some level of RGB was possible from the get go.