I was trying to achieve the same a few weeks ago, except that in my case even the target values for the background color were dynamic too coming from a measure in the same dataset. This case nothing hardcoded could work...
I came to the conclusion that I need to create a custom visual for this purpose, that would ideally inherit from the Line Chart visual. I would add new input fields for the target limits, and fill in the background in the visual myself - while leaving the rest of the line chart as is. It sounds simple, but I couldn't get through the inheritance yet.