There is no super simple way to do this if your colleague already has an AdobeID and Formscentral account. If they don't you can follow the steps in the following document to change the email address associated with your account to their email address and thereby effect the transfer.
If they already have an account with forms they own/authored you would have to make them a co-author of the forms you want to transfer and then before you close your account they would have to duplicate the forms with responses in their account to take ownership of the forms. The problem with this is you will have to republish the links to all the forms you transfer this way.
Hope this helps some.
Thank you very much, Andrew.
Andrew, can you explain how to duplicate the form and responses and transfer to a new owner?
Thanks. This info is hard to find.