Thats not quite true, as far as I have been taught. Humans are in fact coded with the ability to produce green, which isnt a variation of blue and brown. Now Hazel can be a variation of blue and brown or brown and green. Its all on how your pigmentation, density and melanin are coded.
Now any color that isnt blue,green or brown are either a mixture of those or a true genetic mutation.
I’m sorry, it is true. You’ve been taught wrong. If you want to get really technical, even the blue pigment doesn’t exist. It’s the blood vessels that create the bluish color, just as some veins close to your skin appear blue if you have very pale skin.
Humans only have two pigment variations...brown and yellow(or gold, which is dark yellow). Green exists in reptiles and birds, but not mammals.