One of the region-specific Mustang promotions, was the Rainbow of Colors in 1968

These promo paint models could be basic six-cylinder commuters or sporty GT fastbacks and anything in between but only on 500 limited edition Mustangs in West Coast districts including Salt Lake City, Utah; Los Angeles and San Jose in California; and Honolulu, Hawaii. 

A few years later PPG Ditzler put out a supplemental 1967-69 Ford paint bulletin listing 13 colors for a Rainbow of Colors special. 

After collecting several dealer created Rainbow Sale advertisements it also appears from the newspaper advertisements that sale went by many names Rainbow of Colors, Special Rainbow Colors, Rainbow Series, Rainbow Roundup, Special Rainbow Finish, Rainbow Mustang Sale, or just Rainbows.

https://www.hagerty.com/media/automotive-history/7-lesser-known-mustangs-that-are-surprisingly-uncommon

http://mustangattitude.com/mustang/68rainbowofcolors.shtml