If you see that some of your flights on the map are colored purple, it means that you have up to 15 flights on that route in your stats. If you are getting close to having the same flight route 15 times, you will see that this flight will change color on the map from blue to light purple. And when it is the 15th flight, it will change color to purple.
Below, you will find some screenshots of what this should look like on the map.
If you have made closer than 15 trips on the same route, the blue color on the map will change to purple.
When you are approaching your 15th flight on the same route, that flight will turn dark red on the map. When it is your 15th flight, it will turn purple.