If you mark your queens, you should follow the international queen-colour code: White in 2016 and 2021, Yellow in 2017 and 2022, etc. This system has been around for decades because it’s uniform, consistent, and lets a beekeeper know the age of the queen while making it easier to spot her in a crowd. (As you can see from the picture above.)
A few weeks ago, I was showing a hive as part of a field school that I was helping teach for the Calgary and District Beekeepers. I noticed that the new hive, installed as a package in April, had a queen marked in red. I’d forgotten that this year was supposed to be green. Turns out that the queen was from New Zealand and caged in December (mid-summer there), so it was marked red by the Kiwi beekeeper who sold it. That poor queen, young though she be, will always be thought of as a year older.
Queens produced this year should be marked green. A yellow queen in your hive is growing old and a blue or white one might need to be replaced. If you have trouble remembering the order (White, Yellow, Red, Green, Blue), here’s a mnemonic: Will You Raise Good Bees? (Or, maybe you prefer the more graphic: Why your rotting goat barks.)