I've checked my resources, and this hasn't been updated yet, @tcouch. I can see the importance of customising the purchase order and statement templates in QuickBooks Online (QBO). But I've got two alternative methods to take care of this issue. 

First, you'll have a few options to customise your statements in the program. Let's go to the Sales tab in the settings to complete this task. I'll guide you how. 

1. Go to the Settings (Gear) icon at the upper right.
2. Select Account and Settings under Your Company.
3. Go to the Sales tab.
4. Click Statements.
5. Choose either List each transaction as a single line or List each transaction including all detail lines.
6. Check the Show ageing table at bottom of statement box.
7. Hit Save.
8. Select Done. 

The screenshot below shows you the last six steps. For the detailed instructions, see the Customise your statements section through this article: Creating and Viewing Customer Statements. 

For the purchase orders (POs), I'd still suggest adding custom fields and transaction numbers to the forms. You can also enter their default message. Please follow the steps provided by Adrian_A above.

Also, you can run any purchase order reports, such as Purchases by Supplier Detail. It helps you review your POs in the program. For more information, see the How do I run a purchase order report? section through this article: PO Overview.

Second, I recommend looking for a third-party app that can integrate with QBO. This way, you'll have an option to change the colors of the two forms and customise them the way you wanted to. Just go to the Apps menu or through the QuickBooks App Centre.

Here are the steps for the app centre: 

1. Go to this link: https://quickbooks.intuit.com/app/apps/home/.
2. Select AU as the region. 
3. Enter Custom Form in the Search box. You can also navigate the apps from the Categories section. 
4. Choose the right app for your business.
5. Click Learn more/Get app now. 
6. Select the QBO version you're using. 
7. Hit Connect. 

In the meantime, you can visit this website: QBO AU Resource Centre. Then, scroll down the page. In the Help Your Business Thrive section, enter your First Name, Email, and click Subscribe. That way, you'll get the latest tips, advice, and resources straight to your inbox to help your business succeed. 

Please know that you count me in if there's anything else you need. Keep safe always, @tcouch.