If they are greys (darker pits, lighter ridges), they turn brown (rusty-looking) on top when they get wet. The color does not hurt them. If they are blacks (the ridges are black, pits are lighter) then you should wait till they are dark on top. Either way, they sound like they are ready to pick. You want to get them before the snails do!
There have been many studies done about how much a morel grows over time-- I think it averages around 1/4" a day or something like that. Of course, it depends on temperature and rainfall.
By the way, the greys can vary a lot in color. Some mycologists claim the greys and the yellows are the same and the larger yellows are just mature smaller yellows. Other mycologists argue they are different species.