what's the least common eye colour in humans?
The least common normal eye colour is green. Like blue eyes, 'green' eyes don't actually have any pigment of that colour. Unlike blue eyes they contain melanin, which gives a brown or amber appearance, and the green colour is an effect of the 'Rayleigh scattering' where particles smaller than the light wavelength cause the light to scatter. Where there is no darker pigmentation, this causes eyes to look blue; combined with some melanin pigmentation, the effect is green. The exact shade can vary a great deal, as you can see here.
Green eyes are found more commonly in Central Europe and parts of Eastern Europe than anywhere else, but are not typical anywhere.
Even rarer than green eyes are red and violet. However, these only occur with albinism, a condition that makes people unable to tolerate sunlight, so they are usually considered separately from the usual range of eye colours.