Thank you for your post.
At this time, Charting does not have the ability to change the background color of a chart based on the value of a field. I recommend that you enter this suggestion into our Feature Requests web form at:
The entries into this web form populate a database file hosted by Development and Product Management, where the entries are then discussed and considered for future releases. Although I could copy your post and paste it into the web form, there are a couple of questions asked that only you can answer.