I am very curious to lean more about the phenomenological category!

In terms of evolutionary history, monkeys could originally only see the yellow-blue color axis, which is good for perceiving shape under a yellow sun and blue sky. Then a mutation occurred which differentiated yellow into red and green. Look up the 'LMS response' curves in google, and notice how close the red Long wavelength response is to the green Middle wavelength response. Red and green are great for seeing red fruit against green leaves, and also seeing snakes before they bite you. Color processing is complicated. There is the physical light, the perception of the LMS cone receptors and the psychological processing of phenomena. Because the yellow-blue axis was primary, and then red-green was built off of yellow, the artistic complementary colors are different than the RGB complements. A red barn on a green hill is more picturesque than a red barn on a cyan hill. The natural color system is based on painters and artists experience working with color. It is the only space I've added so far that has yellow-blue and green-red opponent axes. I should also note that the original primary colors were not as vibrant as what can be achieved with RBG light. The RBG cube looks so distorted because all of the colors are beyond the bounds of the original system. This is also why you can get negative whiteness and blackness on the gradient tool.

Wow, I didn't expect such a detailed reply! Thanks, this is super-interesting! 👍

Is there a way to make the gradient stepwise rather than smoothly flowing?