FWIW, with the update to R3 27.1.3.1, my 'yellow' has turned to a shade of ... what is this ... brown? Delightful!
Found a way to change colors!
Go to: Reports/Reports and Graphs/Preferences/Colors/Graph and Chart Colors/Custom/Preview
Changing the top two colors will change the colors in the graph.
Worked for me, hope this helps.
Note:
This worked for the Projected Balances Graph, not sure if it will help with others.
New update, same unchangeable colors. Must be a very challenging code change.