Dear Jeanette Mattsson,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
Based on your description, I understand that you have a query "Add color on folders".
According to your description, I really understand that you are referring to the current features in New Outlook preview. And based on my research, I got the same result as yours, there is currently no option to add colors on the folders in new outlook to be easier to sort out mail quicker. I'm afraid many options in legacy Outlook are not all available yet in New Outlook preview, Getting started with the new Outlook for Windows - Microsoft Support.
However, as in Things to look forward to in the new Outlook for Windows - Microsoft Community Hub. Microsoft acquires suggestion from customer and develop them as the new features; in this way we really thank you for bringing this to our attention. I believe your suggestion could make New Outlook even better. We'll bring your issues to our Product Development Team as New Outlook for Windows is still under development.
Please kindly understand that many functions are not designed at present. To get this feature more quickly, I suggest creating an entry as a feature suggestion in feedback collection at Outlook · Community (microsoft.com) and many other users could vote for it. Many features of our current products are designed and upgraded based on customers’ feedback. You can also make suggestions on that website and hope Microsoft will notice this in the future.
I will update here if I get any useful update on this feature. Thank you for your understanding and support.
For other concerns, please do not hesitate to add you post in the Microsoft Community Team.