Open any project in Adobe Express and select Your stuff on the left panel.Select Brands & Libraries and navigate to the library to which you would like to add colors.
Can you share a screen recording of the workflow you're following? It will help us understand the issue better, and then we can assist you appropriately. We'd like to know if you're working on a post, video, or web project, how you are trying to add color & what other results you are trying to achieve.
Please let us know, and we'll be able to help you.