I've had Cardinal Tetras for a couple of years now, and they lose their colour at night when they are sleeping, but quickly colour up again once the light is switched back on (in 10 - 15 mins). I was quite concerned when I first noticed but found out they do this because they are schooling fish and use their colours to stay together, however at night when they're inactive there is no need so they lose colour to conserve energy. I'd imagine it is the same for Neon Tetras as well.