Don't panic! It is just a setting. You're not the first to have this happen to you. Why apple changes it without asking first is a bit unnerving, however you can change it right back.
This link will show you how to change the color back.
iCloud: Change a calendar
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Computer Repair with Care,
Computer Corner
Local Computer Repair and Service- The Villages, Leesburg, and Lake County Florida
(Office) 352-460-1155
(Store) 205 W. North Blvd (441), Leesburg, Fl. 34748
(Website)
http://www.ComputerCornerFL.com