No, the ribbon color on the ArcGIS Online and Portal for ArcGIS websites cannot be customized. In ArcGIS Online and Portal for ArcGIS, the ribbon color for the websites is blue. When working on a project that requires both products, switching back and forth between the two websites can be confusing as there are no distinguishable features.
As a workaround, an information banner can be added to distinguish between the two websites. Below are the ArcGIS Online and Portal for ArcGIS websites in comparison.
The ArcGIS Online website:
The Portal for ArcGIS website:
An information banner can be displayed at the top and bottom of the website to share information with users accessing the website. The style, including the color of the information banner, can also be customized. This article provides an alternative to changing the ribbon color by displaying the information banner on the ArcGIS Online and Portal for ArcGIS websites.
Note: Only members with the administrator role or administrative privileges can add information banners. Refer to ArcGIS Online: Privileges and Portal for ArcGIS: Privileges under the Administrative privileges section for more information.
The image below shows the customized black information banner displayed on the ArcGIS Online website.
The image below shows the customized red information banner displayed on the Portal for ArcGIS website.