@Transparentist
I think it depends on what you actually do in the program. Drawing on a green background with a red 50% opacity brush in paint.net I still get that brownish color. Using a red brush (100% opacity) in additive mode, however, does give yellow.
What I mean is, you’re totally right; Red + Green = Yellow. I was just commenting on how the color combination in your image should be totally yellow but isn’t because of computer BS. :(
@Binkyt11
Actually, Red+Green is supposed to = Yellow, not brown.
The reason why most image editing programs make that ugly brown color when mixing red & green is because of an oversight in how image compression works. See this video from MinutePhysics for an exaplanation.