In reply to Robert Durran:
Like most folks I crank up the exposure when photographing them but if it's a strong storm you will see the colours.
The most vivid I've seen with my eyes were up in Arctic Norway. Basically what you could see was what the camera showed, there was a big swirl of pink and green.
In Scotland the colours are less common but you do still get them. Mostly you'll see a white glow and lines of light with the naked eye.
If it's a big storm though you will see the colours albeit a lot more faintly than in the Arctic.