5 factors...

Mona's Pov:

OMG, there are many reasons to change the colour of eyes, it will definitely help me to understand about the blue-eyed man, let's read everything in detail and understand about it...

I started reading it very excitedly...

1. Age

Babies are usually born with light blue or grey eyes, yet as they grow, their eyes often get darker. This is because eye colour is determined by your genes and the melanin level in your body. As you grow up, the melanin level increases around your pupil, making the eye darker. However, 10-15% of Caucasian eyes change to a lighter colour as they age, as a pigment in the iris changes or degrades.

(After reading this point I ruled out immediately because the blue-eyed man is not a baby and he exactly looks like a hulk, so I can easily strike this point and continued reading to get more information relating to him.)

2. Exposure to the sun