Calendar app changes colour of a calendar on its own I have 4 private calendars in my iCloud account, and I've been using them for the last 2~3  years with no problem.

I think it was last week when I first saw my blue calendar changing its colour to purple. I don't know who/what did it, but I just changed its colour back to blue and forgot about it.

Well today I checked my calendar and it's purple again. I changed it back to blue, but I get a feeling this might go on if I don't do something about it.

The last time I remember doing anything to my calendar settings was exporting the data at the end of last year. Other than that, I use the app everyday to set appts, change event date/time, delete event, etc.

Do you have any thoughts on what caused it or how to fix the problem for good?
The colors would come down from the iCloud servers if you set up a new calendar, so perhaps your device has sync issues.

You could try changing the color using the web interface or perhaps conider disabling calendar sync for a bit on the problem device. Once you're sure the color is set online, you can re-enable calendar sync from iCloud and see if the local version matches the online version settings.

If you don't want this potential change, sign out of iCloud on your Mac or subscribe to the calendars as opposed to syncing them.