How to get symbol callout to match unique line color value

[JacobWallaceAVL]

I have a line feature layer where each bus route is a unique value with its own color. I want to apply a shield/callout style label for each route but I need each route to have the corresponding color instead of a simple white/black background. Screenshot attached. 

[jcarlson]

Unfortunately, data-defined values are not available in most of a label class's properties.

You could create a label class for every color and manually match the label color it, but there's no quick or easy way to accomplish this.