Usually if they are not keeping the blue eyes, then they start to change at about 6 weeks on. My "pointed" kittens actually had aqua eyes - blue/green for awhile. They eventually got green. But they went thru 3 color changes from birth to 3 months old, so the eye color was off for awhile.
I've found that in my cats, after the initial blue/grey, the eyes turn yellowish, and then green later on. Ziggy's turned to gold and now there is some green near the pupils.