I had asked the question about frozen paint to Alex Vallejo, one of the owners of the Vallejo company, since I do live in the Great White North, otherwise known as Canada, and I wanted to know about shipping the paint during the winter, and he told me that yes they can freeze, but once thawed and mixed back up, the fact that they froze should have no effect on them.
If you have a problem with paint freezing and not going back to normal once thawed and mixed back up...contact Vallejo, they will pretty much bend over backwards to make sure the customer is happy.