questions about unlocking

okay, i have a question about unlocking phones. i have a basic idea on what it is (using a phone on a provider that it wasn't originally intended to). my question is, that if you have a unlocked phone, when you go to add it to a provider it wasn't intended for, do they ask you questions?
i'm thinking about getting a blackberry pearl, yet the only colors for verizon (my provider) is silver and pink. but i like the black, blue and red ones. which i believe are only on t-mobile

if someone could help me out with this it'd be appreciated.