It seems to me that the American use of Amber Alert as an anotonomasia from the name of a missing girl in 1996 is an obvious backformative use of an already extant phrase. Amber has always meant "Caution, teke care, go steady, be ready for emergencies" here, from our traffic light sequence of Red Amber Green = Stop Caution Go. In US, the amber tends to be omitted from a continuous on-peak sequence, but left on permanently at off-peak times to indicate a junction where caution would be appropriate ~~ tho, as with most things American in our observation, there will be State &/or regional variations.

~Michael~