The three wavelength sensitive cells (peak sensitivity in the red, blue and green) of the retina provided three color axis coordinates, but just like Cartesian coordinates can be transformed into Polar Coordinates, these initial coordinates are transformed (in or before V4) to an axis that is mainly intensity, a blue/yellow axis and a red/green axis. It is more complex than I remember how to explain now, but crudely speaking a lot of activity is some cells making up the red/green axis compared to other cells of that same axis will be perceived as red (or green if the other cells of that axis are more active.) You probably have stared at a red shape for a few minutes and then looked at a white wall and perceived a green spot of the same shape. This is because white light normally stimulates the "red cells" of the red green axis equally to the stimulation that it gives the "green cells" but after staring at the red spot for a few minutes, the red cells are fatigued and do not respond as much as they normally would to white light - I.e. there is a net excess of green cell activity so you perceive green spot on the white wall. Possibly some mix of red and green light might be perceived as sort of white or "dingy", but there is no perceived Reddish Green color possible. I suspect you are confusing how subtractive pigments combine to make white light reflecting from the combination of two pigments have quite a different perceived color. Blue and yellow water color pigments do make a nice green. Brown, may be the perceived "color" of red and green subtractive pigments, but unlike green, there is no pure wavelength that will be perceived as "brown."