I also heard the people who have grey eyes may have different eye color in their later lives. But this kind of change is due to the increase of the amount of pigment. And it won't happen in one day. I know that the people who have black eyes when they were young, they may have brown eyes when they are getting older. And if the color of the eyes changes dramatically within a day, this person may suffer from serious disease and need to see an eye doctor immediately.
The color of our eyes will change slowly when we get older. Usually they change the color because the pigments in our eyes increase. Grey eyes have the least amount of pigment when compared with other kinds of eye colors such as blue eyes, hazel eyes or black eyes. So I think your baby may have many kinds of eye colors in future. But this won't happen in one day.