Rs7495174  A or G.

AA tend to have blue eyes.  AG or GG tend to have brown eyes.  This polymorphism accounts for 74% of variation in human eye color.

This polymorphism is in OCA2, the oculocutaneous albinism gene (also known as the human P protein gene).  This gene is involved in making pigment for eyes, skin, hair. If OCA2 is not expressed in the eyes, but is expressed in the skin and hair, then one has blue eyes. If OCA2 is absent in eyes, skin and hair completely, then one is an albino.

There are at least eight other genes involved in eye color, but OCA2 plays the largest role.

The Tech museum has an eye color calculator to try.

More info on eye color from HudsonAlpha Institute.