It’s worth noting that not only have humans never evolved green or blue hair, neither has any other mammal (at least that lives today). Green especially seems like it would be a great camouflage in some environments (green snakes would agree), so I wonder if there is something about hair that doesn’t ‘hold’ green or blue pigment well?
I suspect this thread will be merged, but I’ll respond here anyway.
Hair colors black through the lightest blond can be provided by varying the amount of melanin - that’s 1 pigment only. Red seems to come from some other mutation.
For hair to be able to be green or blue, it would likely require a significant mutation to provide an alternate pigment. As the column said, maybe that would require more than one mutation to work in conjunction, so is unlikely.