Can I reassign the client account? I would like to give my client the client account, but first, I would like to assign my colleague as the client, just so we can see what the process looks like and see how our client will view Craft. Will it be possible to update the name and email address afterwards?
Sounds like you just need to initially setup the client account with your colleague's email/password/name.

Have them do their testing and when you're ready to give it to your client, either just update the email/password/name or delete the client account and re-create it for your client.