Supply PowerBI Report Server (not SRSS) default colors.json file for branding
Typically you want to just tweak a few things from the default, e.g. just a company logo. So you would need the Microsoft default colors.json as starting point.
There is a default for SRSS available, but not for PowerBI report server.
Hannes Pernevi Lindgren commented
This! I render the Power BI report Server Branding option useless without the possibility to change single elements only. Either allow branding to affect only single elements or provide a default colors.json. (Add: Also we cannot upload a company logo without a complete colors.json.)