The option to customize the color of your bank register is currently not available in QuickBooks Online (QBO). I can see having this option would be a great additional feature in the feature.
Rest assured our Product Developer team is always working on finding ways to improve QuickBooks. This way, our users such as yourself can seamlessly continue with your business. To stay current on the latest news and updates about QBO, visit our QuickBooks Blog page for reference.
Additionally, let me also this article that you can read for future guidance in reconciling your account: Reconcile an account in QuickBooks Online. This link can walk you through the steps in effectively matching your accounts in the future.
Know that you’re always welcome to get back to me if there’s anything else you need help with in managing your accounts. I’m more than happy to help you. Have a good one and keep safe!