The recent red vs blue is an invention of the news media, and the usage has flip-flopped several times in the past. It was intended only to give the networks a way to show which party was ahead in their coverage of elections, and the current usage as to which party is which color has only appeared consistently in a couple of recent elections. But people who like simple-minded "tags" for everbody (makes them easier to hate) have picked it up to some extent as a party designation.

In the era of the House UnAmerican Activities Committe (HUAC - The McCarthy Committee) "reds" was used by the public interchangeably with "Communists." It might have been originally an "insider code" used by party members in conversations that might be overheard, but came into general use quite rapidly once the witch hunts got started.

John