Eye color is the result of variations in the amount of melanin, a pigment found in the front part of the iris of the eye. The lack of this pigment results in blue eyes, some pigment gives green and lots of pigment gives brown eyes.
So light brown eyes just have a bit less melanin than darker brown eyes. All of the different shades of eye color happen the same way. Blue-green eyes have an amount of melanin between green and blue, hazel eyes have an amount of pigment between green and brown, etc.
