Hi Jamie,
ArcGIS Online and ArcGIS Enterprise do not support changing the color of the Content page banner.
However, there is a feature called the Information Banner that is available in both ArcGIS Online and ArcGIS Enterprise that can help differentiate between environments. When configured, the information banner appears across the Home, Gallery, Scene Viewer, Groups, Content and Organization pages.
Here is some information to help get started with the Information Banner:
Thanks,
Marley