Red has several usages, some may not be evident to someone outside the U.S.
Red flags or red lanterns mean danger, or stop.
'Raise the red flag" was originally used to indicate readiness for battle.
Red is used to indicate hot temperature, red hot passion, etc.
"Red" applies to Communists, as noted by McGrath.

Wiki notes that 'red' was applied to Republicans in 2000; on election night all broadcast networks used that color to mark Rebublican election results. The GOP uses a red logo on its website, but it has not been officially adopted.
Blue was used for Democratic results.

A red light is often associated with a house of ill-repute.

Red is associated with the Devil.

Occult meanings.