Since all of the residential colleges offer similar facilities and a high standard of pastoral care, we recommend that you accept any offer received, even if it isn't from your preferred college. You may not necessarily receive another offer in time for the start of semester.
If you decline an offer you are placed on the waitlist. However this doesn't necessarily mean you will be at the bottom - the waitlist is divided between male and female and does not have a numbered priority of applications.
Once you accept a college's offer and pay you are in a legal binding contract with them and no other college can subsequently consider your application.