http://www.eyedoctorguide.com/eye_general/eye_color_genetics.html
There's a chemical in your body called malanin. It determines pigment (color) of eyes, skin, and hair. The "Default" is brown; if your eyes are the usual "turd brown" color, then you have the appropriate level of melanin in your body (evidence by brown being the most common eye color between races, and even species). A deficiency causes lighter eyes, skin, and hair. This makes your eyes blue (which is very low melanin), your hair blonde, and your skin pale.
Melanin-rich Brown eyes often have a gold tint, like mine. People with low melanin levels have blue eyes. Blue+yellow=green. This is a horrible over-simplification, but i think it gets the idea across.