Spectrum x Spectrum Blend

I made the plot above by averaging the color from the spectrum on the x axis with the one on the y axis, and as you see in the circled portion blown up you get some non spectrum colors such as brown. I don't know if this is every fully saturated color or if you get more colors adding another axis...
I'm thinking if you have two prisms shining their rainbow on a piece of white paper, there might be a way to hold them so their rainbows cross to make every color...
I'm not sure the cause of the roughly 3x3 grid that appears, whether that is because humans have 3 color receptors or because computers add 3 colors of light to make every color or both...