I'm having this issue as well, I'm just posting now to add to the demand for a fix. It seems like there is a "workaround" but a fix would be much more appreciated for others who are experiencing this issue.
Adding my voice to the noise! Also have a problem with my green calendars suddenly becoming purple - it sounds trivial but it's REALLY annoying when you organise by colour. It's also across ALL of my devices - phone, macbook, imac and ipad. Apple, please help!
Not working here too. The defaults calendars (HOME and WORK) keep changing colors to its defaults BLUE and PURPLE. All devices : MBPro, iMac, iPad and iPhone. Renaming is working fine, but changing colors not !