Thanks for your feedback and the heads up about our certificate. We've recently been doing some behind the scenes work for an upcoming update and have now fixed the certificate issue.
As for creating an account, we decided to do it this way so the user's paints and lists are synced to the server. If you lose your device, you can simply login to a new one and everything will be synced back over. It also allows us to add new paint ranges without an app update.