iCal changing home calendar color on its own

I use a MacBook Pro laptop running OSX 10.7.5. I have iCal on my laptop and iPhone and back up to iCloud. My calendars have been running flawlessly for several years. Today, all of a sudden and without my doing anything, the color of my home calendar changed from red (which it has been for at least two years) to blue. Reminders changed from blue (where they have been for some time) to orange. All other calendars stayed their correct colors.

I changed the colors back to their original colors, and within 10 minutes they had changed back again.

I quit iCal, restarted my computer, and changed the colors again when I got iCal back up. Within a few minutes they were back (to blue for home and orange for reminders). It's not a big deal, but just annoying. Why would this happen?