Marking a queen helps you to locate her so that you can safely transfer resources to other hives.
It is only a purpose to the color used to mark a queen! While you can purchase whatever color you wish beekeepers use the following colors to track the age of their queens.
Years Ending in #1 & #6: White
Years Ending in #2 & #7: Yellow
Years Ending in #3 & #8: Red
Years Ending in #4 & #9: Green
Years Ending in #5 & #10: Blue
