Why are Republican states colored red and Democratic states colored blue? Answer: because Democratic states were colored blue in 2000, and the long recount that year seared the electoral map into everyone's memory. After that, the association became permanent.
Fine. But why were Democrats blue in all the 2000 maps? After all, red is traditionally the color of lefty parties around the world, and before 2000 elections network maps had usually colored Democratic states red. Several years ago, one of my commenters provided the answer:
Since the advent of color TV, there has been a formula to avoid charges of giving any party an advantage by painting it a "better" color. Here is the formula: the color of the incumbent party alternates every 4 years.
I've never gotten ironclad confirmation of this rule, but it seems to be correct. The table on the right shows how this formula has applied since 1976, and it explains why Democrats had usually been colored red prior to the 2000 election: it's a coincidence. In the six elections prior to 2000 every Democrat but one (Dukakis in 1988) had been coded red, but that was just because of how the cycle of incumbency happened to work out during that period. If the formula had continued, the incumbent Republicans would have been blue in 2008, but by then it was too late. The color of the parties had entered American folklore and become permanent.
HOLD ON!: In the Smithsonian, Jodi Enda dives in deeper and calls my story a "myth." Map colors weren't monolithic among the networks, and she finds no evidence of any kind of rule for how the colors switched. Rather, in the early years of color mapping on TV they "changed back and forth from election to election and network to network in what appears, in hindsight, to be a flight of whimsy."