And as an utterly hilarious note of irony, silver dragons were actually left out of 4e's first Monster Manual because of this shift towards EVERYTHING IS EVIL NAO!
Funnily enough, even 2e had possiblities for good red dragons and evil silver dragons in many of its supplementary material. They were in fact called 'rogue dragons', and were detailed to a degree in Council of Wyrms and 2e's own Draconomicon.