Question

why not multi-colored (rainbow colored) eyeball possible in humans ??

Answer 1

The colour of eye depends on the amount of melanin pigment present in iris which is a coloured portion of the eyeball and is shaped like a flattened donut. Dark irises have dense deposits of melanin. Melanin pigment absorbs most wavelengths of light and therefore very little incoming light is reflected from very dark eyes. If the colour of eyes is blue, green or grey that means many of the wavelengths that enter the eyeball are reflected out. If the iris is multicoloured then all the wavelengths will be reflected out. Hence, the person will be blind inspite of having perfect eyes. Thus, multicoloured eyeball is not possible.