I always thought it was green over a certain % battery level, then yellow, and then red under 15%.
This is how it works. I have two S7 Edge phones sitting here. One is 100% charged and the graph is green. The other is at 10% and the graph is red.
Both phones are on T-Mobile.
I have a Note 5 at 86%, the graph is green. Note 4 at 14%, the graph is yellow. Note Edge at 57%, the graph is green. All these phones are also on T-Mobile.
It doesn't have anything to do with the signal strength.