I'm going to agree with you that there are too many distinctions and that grey eyes are a form of blue and amber eyes are the lightest shade of brown however your theory of blue, green, hazel and brown being enough is incorrect as true hazel eyes are just simply a light golden brown (slightly darker than amber) and the definition you've given hazel eyes (green and brown) makes them fall under the green category. So I say there are only 3 true eye colors; blue, green and brown, the rest or significantly different shades/mixes of the 3.