You should really find a vet opthamologist. I know someone with a cat like
this. I don't remember the cause but for her cat it is just a pigmentation
change however, they watch it in case it should show signs of being cancer.
His has gotten slowly browner over 2 years. It's very noticeable in photos
now.

I second what Karen said--try a vet opthamalogist. Your vet will
probably refer you to one anyway. I decided to do this with Benny's eye
problem and I'm glad I did.

Rene

Lighter eyes can get freckles. I have some in my eyes. My eyes started off more
green, now I'm getting brown flecks. See pic below

http://www.mary.cc/images/eyeballing.jpg

I just have to watch them to make sure they're not cancerous, and wear sun
glasses.

I've not heard of the eye color changing evenly. Is it turning whitish?
Blueing? any bulging? secretions? Any signs of vision loss?

As others have said, have the eye checked out by a vet.  It could be
disease, or it could be odd pigmentation.  My mother's cat had one
yellow eye, and one that started out yellow but eventually turned
brown because of pigment.

It sounds like the iris pigmentation could be "melanosis." Our late
tabby had this condition. One specialist suggested immediate removal of
the eye because of the risk of cancer. We got a second opinion from
another opthamalogist. He examined the eyed and determined it wasn't
cancer, although there was a risk that it could become cancerous
(melanoma) in the future. He said that the eye didn't need to be
removed unless the pigmentation began to thicken (or become raised).
Instead, we just had to bring out cat in every 3-4 months for an exam
to monitor the eye. Our cat died three years later of unrelated causes.