wpa2000, on the iPhone 6s Plus I have the battery line will turn to Red when the battery level reaches 20% or less just to indicate the battery is getting low. Perhaps yours shows up in Yellow, or we're color blind
SuzyQ, good pick up you are correct. Had never used the Low Mode but just tested the preference in the 6s+ settings and it will turn the battery indicator to yellow. So the red color is just to indicate when the battery goes to 20% and lower.