Hi Mike,
Thanks for your interest in Syncfusion products.
I hope by background, you mean the line segment color. Please refer to the sample from the below given link, that illustrates a chart with custom line template. This template uses DataTriggers and check for values 110 and 140 and sets blue color.
http://files.syncfusion.com/support/Chart.WPF/7.3.0.6/84613/main.htmPlease let us know if this helps. Also please let me know if I have misunderstood your query.
Regards,
K. Saradha Devi