Colour Change In Fish

Hi all just wondering... I came home from work today and as I was home about 4 hours later than normal my tanks lights were not on yet. I'm getting some timers at the weekend to turn th rlights on automatically. But my question is when I switched the lights on like always I have a look round the tank make sure everyone is there and looking fine etc etc I noticed today that my tetras and tuxedo platys looked dull and almost no colour to them it got we worried they were just sat at the bottom of the tank not doing much then with ink maybe 5 minutes the colour came back and they were buzzing around the tank like normal and even more after they had their feed, is this normal? I've never noticed it before so was concerned when they looked a bit off!