I am having the same problem on my iPhone (iOS 7.0.2) and now my iCal on my Mac (OS X 10.8.5), too! I have always had my "work" calendar black and now it's purple. Black is not even a choice. But even when I change it to brown, it flips back to purple. Very very frustrating. I've had these issues with subscribed calendars before but never my own iCloud calendars. This appears to be a BUG, not something that a user can fix???
I am having the same issue... My work calendar is and always has been yellow. This morning it is purple and when I choose yellow or any other color it isn't sticky and defaults back to purple.
Several other people have reported this in other threads and threads for other OS's...
Yet another strike towards iOS 7...
I depend on my iPad and iPhone to function. I depend on the ability to see my calendars in different colors. I am business owner, and adult with ADHD and the iPhone with location based reminders helped me and profound ways.
I have 8 different shared calendars, all with different colors. In addition to the calendars changing back to purple twice, I had 2 appointments that I created on ical on my mac, not show up on my ipad or iphone. This resulted in a missed appointment. Apps are crashing, things are buggy, and the color is too bright. This so un-apple like. Everyone I talk to about iOS7 has a great deal of contempt for Apple right now. Please, please, please fix these issues.
I just had the same problem. I went to the iCloud to change the colors back, but they would automatically change back.
Here's what I did:
1. Take your calendars offline in iCal
2. Export the calendars that change color to the desktop
3. Delete the calendars from both the iCloud and iCal
4. Create new calenders in iCal making them the color you want
5. Import the calendars from the desktop int your new calenders
6. Put your Calendars back online to sync with iCloud
I did aguest's solution and it worked but then I found an even easier way. Apparently it is an iOS-MacOS issue. Make sure all your calendars are synced through iCloud so all data on all devices is the same before doing this.
0. Although not necessary, why not export your calendars to your desktop just in case something goes wrong: in iCal on your Mac click Calendars in the left top corner, select each calendar in turn, and select File, Export
1. On your Mac, take Calendar off-line: in System Preferences select iCould and uncheck Calendar. A pop-up window will appear that asks confirmation you want to delete all calendars from your Mac. Click Delete and all Calendars and Reminders that were synced through iCloud will be deleted from iCal.
2. On your iPhone or iPad go to the Calendar app. At the center bottom of the screen tap Calendars, tap info (the i in a circle) next to each calendar in turn and change its color. When done click Done to exit the Calendars pop-up
3. On your Mac in System Preferences select iCould and recheck Calendar. A pop-up window will appear that asks confirmation you want to merge all calendars from your Mac with calendars in iCloud. Click OK and all calendars will be synced through iCloud.
4. Now you can change the calendar's colors to your heart's content from any source: iPad, iPhone, or Mac.
Unfortunately this solution doesn't seem to work for Reminders with sticky colors. But for Reminders just create a new Reminder list and move all items over to the new list (easiest way: export the old list to your desktop, create the new list, then import into the new list). The new list will allow you to change color.
Hope this helps.