Is there an actual statistic to reflect how rare it is to have identical twins with DIFFERENT eye color?

We have identical twin girls. One has hazel-green eyes, the other's is blue. We are certain our girls are identical, so please do not insist that they are fraternal. Besides their eye color and a little difference in weight, they are both fairly identical. Even I sometimes confuse them in photos. Anyway, I know the occurance of diverse eye color in identicals is rare, I'm just unsure if there is an actual number ociated with it.