I once heard Whit Gibbons sum this up better than anyone else. He said if you were using the red against yellow to tell these snakes apart, you had no business going near them.
This rule does generally work in the southeastern US. However, in the western US there are snakes that have the red bands touching the yellow that are not venomous. In the US, there is no venomous snake with the red bands touching the black.
And, as you have correctly pointed out, many of the coralsnakes that occur in central and south America don't follow this rule. There are coralsnakes with no red, coralsnakes with no yellow, coralsnakes with double bands, coralsnakes with no bands, etc. Then there are harmless snakes that have perfect "coralsnake" patterns and every other pattern a coralsnake can show as well.
The point is, this is not a reliable way to tell coralsnakes from non venomous snakes. There are much more reliable ways. Head shape is a good place to start for milksnakes vs coralsnakes.
However, some of the central American snakes in the genus Pliocercus are shaped and colored exactly like coralsnakes!
-----
Chris Harrison
Central Texas