Most rainbowfish do not go dull and they are not nocturnal (except blue-eyes, I believe) . In fact, they are at their brightest in the very early morning when the sun (tank light) comes on. That's when they exhibit their breeding colors and spawning behavior.
Gorgeous sight first thing in the morning, lemme tell yah. If I remember I'll try to get my camera set up at 4am and take some shots.
They pale out during the day a little, but still display their colors off and on.
Roan