Hi Paul,
This is a question we have received before on charts and assigning specific colors to specific values. Currently, there is not a native way to fix a specific color to a specific value in our charting as they attempt to adjust on the fly to any changes. Whether it be the addition of new categories or the removal of categories.
I would encourage you though to include that feedback in our User Voice as a business need for your company. UserVoice is our Feedback Platform- which can most easily be accessed from the My Apps page in Quick Base by clicking on the orange Feedback tab that appears on the left hand side of the page or at
http://quickbase.uservoice.com . This forum is used by our development team to explore customer suggestions for enhancements / changes to the platform.