My 14 month old, Finn, didn't lose his baby grey/blue tinge and go brown until about 7 months old. And like Macabre mentions, he still has a blue ring, but the rest is brown. I don't think it's anything to be concerned about at all, it's not like a true blue eye/glass eye. I think some just color change faster than others.
Sparta kept his green/grey/blue color untill about 2 weeks ago now they are slowly turning that pretty chocolate color and he is 3 months now. I loooooved the color they were lol wish it was possible for him to keep it lol