Instead of red how about nothing? After all a dead radar and no radar are functionally identical (unless you count marking the radius of what will cause a base to flash, but then isn't that a separate setting that is usually set equal to the radar ring).
I would make one more suggestion: make the radar shading a union Venn diagram, not an intersection one. I don't see any advantage to making the areas covered by two or more radars successively darker, and on terrains that have lots of overlapping radars, like Tankland, the multiple overlapping radars cause the map to go almost completely black.