I'm looking for an algorithm to do additive color mixing for RGB values.
Is it as simple as adding the RGB values together to a max of 256?
(r1, g1, b1) + (r2, g2, b2) =
(min(r1+r2, 256), min(g1+g2, 256), min(b1+b2, 256))
It depends on what you want, and it can help to see what the results are of different methods.
If you want
Red + Black = Red Red + Green = Yellow Red + Green + Blue = White Red + White = White Black + White = White
then adding with a clamp works (e.g. min(r1 + r2, 255)) This is more like the light model you've referred to.
If you want
Red + Black = Dark Red Red + Green = Dark Yellow Red + Green + Blue = Dark Gray Red + White = Pink Black + White = Gray
then you'll need to average the values (e.g. (r1 + r2) / 2) This works better for lightening/darkening colors and creating gradients.