Apple Watch Run Workout Map Route

After finishing run and checking details in workout app, route shows up with red, yellow, and green. I have ran this route several occasions and my ending race time has improved by about 4 minutes!!! However the line colors from route map have now turned to yellow and orange for the most part. Back when I first started running the course it was mostly green and yellow. Do this colors mean heart rate? Pace changes? Just confused how my ending time is better but map route color shows slower pace during run?????