Script processing normally occurs at the end of the document load. In Chrome, adjustments to the data stripe color need to be done earlier in the process. In the Tampermonkey editor, with the Port Monkey script open, there is a tab for Settings. There is a setting to "Run at", and changing that to "document-body" will allow changes to the data stripe to occur.
I did notice that this changes the way the trade confirmation windows appear, from what I had set up in Port Monkey. It may change other behaviors that I did not discover in my brief tests. BUT, I also noticed that -- once I have the data-stripe color that I want -- I can set the "Run at" back to "default", and the adjusted data-stripe color continues to work.