It has to do with dominant and recessive genes. The most dominant gene is Brown. But, both blue and green are recessive genes.
When the blue and green are the only genes present, they are actually co-dominant, so they could take on blue, green, or any mix of the two (that's how we get other shades of green/blue).
I've also heard they are a mutation of brown/blue eyes and it depends on how much melanonin is present they can mutate into green. I don't know if that's true though.
I have green eyes and some of sibling do as well. I also have blue-eyed siblings as well as brown-eyed siblings.
My father's eyes were brown and my mother's were blue. I think depending on what genes each parent carries from their ancestors, they can have children with any color.
