Blue eyes don’t have blue tint in them, in fact they are transparent. We know that brown eyes get the colour from the same pigment that also colours our skin, ‘melanin’, but blue eyes get their colour from the scattered light in the Iris.
The coloured part of the eye is called Iris. This Iris composed of two layers. The front layer (called the stroma) is made up of overlapping fibers and cells. While the back layer (called the pigment epithelium) is where the colour pigment is.
If there is no pigment at all in this front layer, the fibres scatter and absorb some of the longer wavelengths of light that come in. Hence more blue light gets back out, and the eyes appear blue.
The variation of colour in eyes happens when different parts of the Iris have various amounts of pigment in them.