"When I try to visualize 3 types of sensors (without left hump), I get a straight line not a triangle" So where is magenta on your line? This shows all hues,

"When I try to visualize 3 types of sensors (without left hump), I get a straight line not a triangle" So where is magenta on your line? This shows all hues, and obviously magenta has a place on it, even though it is non spectral. https://www.allthesky.com/articles/preview/rgbtri.jpg https://www.allthesky.com/articles/preview/rgbtri.jpg It doesn't seem like you are really trying to visualize the input from 3 sensors if you are putting it on a line, you are still trying to represent wavelengths. Otherwise you won't be treating each of the 3 sensors equally.... you'll be putting two of them at the ends, and one in the middle. Which one goes in the middle? Maybe it would be better if you just think of it as a color cube, which includes black, white, and all shades of gray. It is a cube because that is the best way to geometrically represent 3 dimensions. https://www.pngitem.com/pimgs/m/592-5920896_rgb-color-model-cube-hd-png-download.png https://www.pngitem.com/pimgs/m/592-5920896_rgb-color-model-... (the triangle is a cross section of the cube.... the one that includes the red, green and blue corners)