

Glad I could help.
That is true, and I get that it should be simple for children. But that doesn’t mean that the foundation has to be incorrect.
Just using the simplified version of a correct layout like I showed you should be the way to go in this case.
Of course, most people won’t need to know what’s colorsystems there are. Itten is none of the less still teached by artschools even though it is this incorrect simplified version of a color space. At that advanced stage, there is no need to stick to a simplified version, let alone a one that doesn’t lead to correct results.
Ittens model is just a remnant of its time. And it keeps being shared because of that simplification. But hey, that how history sometimes goes.



(Upper is correct, lower is Itten)











Gouache can be lifted of the canvas because it has some chalk like texture. But it depends on the paper quality, how often you could do that.
Nonetheless this is still a lot more difficult than digital.