The color of my calendars keeps changing

Recently the color I selected for my calendars keeps changing arbitrarily. If I edit it back to the colors I want it just flips back to other colors I never selected. This is happening across my devices whether I set the color on my Mac in iCal or on my iPad. It is really annoying. I've had the same colors set for years with no problem, now suddenly this is happening. What can I do?