Thanks to a very nice fellow fan at IMDB, who comments on my post on Martin's actual eye color. By far, she is the only one who managed to give me a 'satisfied' answer. So kudos to sadfilmophile, turns out that dear Martin's eyes are all blue, grey and brown.
Image source: http://jessamygriffith.tumblr.com/post/23450124660/let-us-consider-ladies-and-gentlemen-martin-freeman
I honestly thought it's normal for caucasian eyes to have difference colors in their eyes. So, there is an actual scientific term for it. And it's a condition called 'central heterochromia'. Therefore, the outside part of the iris is mostly blue + grey and the ring around the pupils are brown with specks of gold.
And also, under different lighting, the eyes will seem blue this moment and then brown the next and sometimes even grey. According to a quick search done by sadfilmophile, someone actually calls Martin's eye colour 'magic'. <3
Coincidentally, his other 'half', Benedict, also has the same condition of partial heterochromia.
Benedict's eyes are a lighter shade so it's more obvious. Yup, there you have it. =X