It's generally accepted that there is a gene for all eye colors. This article makes it seem like a blue-eyed person is just lacking something, but in fact it's a genetic mutation. I'm very surprised at the date, and we'll see if it stands up over time.
I don't know when green eyes first showed up, but they are a distinct color - not a mixture. I, my wife, three children, and one grandson have green eyes. Seems to predominate in people of Scottish descent.