Eye, hair and skin colour vary around the world because of the production of a pigment called melanin. One gene in particular, HERC2, is one of the genes responsible for melanin production and its effect can be seen directly from your eye colour. Your FitnessGenes result tells you which version of this gene you carry and what level of melanin you are likely to produce as a result.
Eye colours can range from the darkest shades of brown to the lightest tints of blue. When people talk about eye colour, they are talking about the part of the eye called the iris. The iris is actually responsible for regulating the amount of light entering the eye by adjusting the size of the pupil.
The front layer of the iris contains a pigment called melanin. It is this pigment that is responsible for what colour our eyes turn out to be. In general, an iris containing lots of melanin will look dark brown, as any light hitting the front of the iris will be absorbed. In comparison, if there is only a little melanin in the iris light can pass right through and reflect off the layer of melanin at the back in such a way that the reflected light looks blue. This is actually the same principle that makes the sky appear blue!
What does eye colour have to do with fitness? Very little to be fair, and this trait is more for fun. However the production of melanin, and how much we produce, particularly in our skin, does impact on how likely we are to be deficient in a key vitamin - vitamin D.
Source: 1000 Genome Project. Global averages for both sexes