I'm an Iridologist. Without seeing your eyes, it's really hard to know for sure. If the brown marking are near the center of the pupil, they can be signs of digestive weakness and certainly doing juice fasting combined with colonic irrigation can help to lighten those marks. But, you may have been born with digestive weakness or perhaps you were not breast-fed as an infant leaving you with an imbalance of good bacteria in the gut and so you may need to accept some of those imprints and not make your end goal to have blue eyes. You can be extremely healthy with pigmentations in your eyes and with a mixed eye color too.
In Iridology, there is no green colored eye, so what you are speaking of is most likely a blue eye with a yellow hue. That yellow hue can make your eye look green. It can be a sign of kidney weakness or elevated uric acid levels in the body. The short solution to that is: drink more water and avoid processed foods.