"Melanin" natural substance in the body that gives a variation in the color of our eyes.

From brown color to green color depends on the amount of melanin in the iris" which in turn is determined genetically. 

There are three genes that affect eye color, so it’s hard to pinpoint the exact color that a child’s eyes will be. Individuals with black or brown eyes have eyes with more melanin to block the sun’s rays. Those with blue, green, or hazel eyes have little protection from the sun and may experience discomfort, irritation, burning

Most human eyes are brown or blue, but other colors like hazel and gray are more rare. 

Blue eye