While I speculated it was because of the water reflection or the contacts not working in the water and the colour being changed post-production and that edit hadn't happened yet, it seems there is another possibility.
What doesn't make sense to me is why they would do the hair, wardrobe, pale make-up and everything.... except the contacts....
I mean, if it was an oversight, say so and just fix it for the movie right?
I don't know, and I don't care... they know now and will fix it for the movie ha ha - for now we can look at Rob & Kristen's real eye colour and imagine it is just a photo from their day off, ha ha!
Read the Interview here