There's a mutant in the woodpile.
More seriously, I guess the green and hazel result from mixing the DNA of brown and blue eyed mates. Maybe someone with better knowledge will weigh in.
Hazel eyes like mine are about what you say, instead of having the code for blue eyes or green being homogenous across the iris you get a hodgepodge of green coded iris cells and brown coded iris cells equally interspersed across the whole iris in varying degrees, sometimes hazel eyes appear close to blue or green while others appear close to brown or amber. In many people with Hazel eyes you get different shades and different colors. My eyes for instance are green and blue with a gray backwash and one brown/black spot. There isnt any real genetic study to explain hazel eyes out there.