When you know someone has blue eyes are noticing what?

The color of their eyes, specifically the iris. The iris is the colored part of the eye that surrounds the pupil. Different people have different colored irises, and blue is one of the most common eye colors. The color of the iris is determined by the amount and type of melanin in it. Melanin is a pigment that also gives skin and hair their color. People with blue eyes have less melanin in their irises than people with other eye colors.