Here is an RGB image. RGB is additive color. Can you guess how to mix the RGB color separations to produce the image?
You add the three color separations together. In Photoshop, the blend mode for adding is LINEAR DODGE.
(You can also use SCREEN in this case, since the R, G and B channels are completely independent of each other, SCREEN and LINEAR DODGE produce identical results)
The layered .psd file is here if you want a look.
Next up: (so-called) subtractive color mixing. Can you guess which blend mode combines CMYK color separations?
