Whereas people with brown, blue, and green eyes usually only appear to have one color in their iris, people with hazel eyes have several, according to an article in Owlcation. Additionally, as previously stated, having hazel eyes is less about color and more about how color is distributed within the iris. And that can have some significant variation, as there's a lot at play when it comes to how hazel eyes appear.
Specifically, the hazel pattern begins with a ring of color around the pupil, usually brown. Then, as you move away from the pupil and out toward the rest of the eye, the color will shift into green, sometimes with an additional ring of amber in between. This burst-like pattern is what makes hazel eyes distinct from green eyes, which don't have the same kind of color shift and are monochrome in appearance. As noted by Owlcation, hazel eyes "may have a yellowish brown, dark brown or amber-brown surrounding the pupil."
That's not to say that people with different eye colors can't have variation in their irises, as they certainly can. But this unique variation is how it works for our hazel-eyed brethren.
