The RYB delusion
When I learned about colors in primary school, I was taught that the primary colors are red, yellow, and blue. I would bet that you, reader, were also taught in this way. If you do a google image search for 'primary colors', you'll find plenty of images with the specific colors shown above. I am here to argue that we have all been lied to, and that the only correct, canonical, truthful and logical primary colors (for pigments/paint mixing, aka subtractive color) are cyan, yellow, and magenta.
Part 1: Green Failure
The main failure of the RBY system is that it is impossible to make a decent green with pure blue and yellow. The reason is that pure blue and yellow are actually complimentary colors. If you remember learning about colors from grade school, you'll know that mixing complimentary colors results in brown or grey. If they are perfectly complimentary, they should cancel each other out and make a pure grey. Below, I will make your browser generate a gradient starting at yellow and ending in blue.
As you can see, a mix of yellow and blue creates grey, not green, because they are complimentary colors. Now, here is a gradient from yellow to cyan instead:
Wow, that is a much different result, and it is actually green.
In my opinion, the main source of this issue is that most people do not differentiate between cyan and blue, and just think that cyan is a shade of blue. Maybe the idea of "blue" to some people is somewhere between cyan and blue, like this:
This blue is half-way between blue and cyan. It's not perfect, but it's close enough to cyan to be able to produce some green when mixed with yellow.