ICloud holiday calendar appeared in Calendar app

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.

You can not sync a Google calendar and an iCloud calendar out of the box. (Google and Apple both seem to avoid making it possible, probably because they both try to pull users onto their side.)

However:

1. All Apple devices can use and sync with Google calendars additional to iCloud calendars (I use it a lot with >10 Google calendars from >5 different users)

2. I saw some syncing tools that keep iCloud and Google calendars in sync, but a) they have to be always running somewhere and b) they are usually hacky and unstable.

You can Google both; I recommend 1.