There are five queen bee marking colors that follow a internationally recognized color sequence depending on the last number of the year that the queen was born. Since queens do not live more than 5 years, the color code starts over in the sixth year. A common mnemonic used by beekeepers to remember the colors is, "Will you raise goods bees".
It is common practice to mark the queen with a small spot of paint on her back (thorax). A color code exists within the beekeeping industry to indicate the year the queen was introduced.
Model car paint or POSCA paint pens may be used to mark the queen. The identifying mark should be small, so that it does not cover any other part of the queen. A 1/16″ stick, lightly dipped in paint, is a good applicator. Generally, queens are marked before being introduced, but they can; however, be marked at any time. Paint should be given ample time to dry before the queen is released into the colony. In fact, queens may be purchased already marked by the queen producer. (Original text copied from Maine State Beekeepers Association, sourced from ohioline.osu.edu)