Angels frequently losing colour?

Is this fairly normal? I have 2 juveniles in a 35g tank who have a habit of rapidly losing their dark solid black bands in a matter of minutes, only to have them back a few hours later.
The tank is moderately planted and generally peaceful, they get the very very occasional fin nip from my new yo yo loaches during feeding time, but apart from that nothing bothers them at all. Sometimes I wonder if its just the way I see them in different light, but there are definate changes throughout the day. Ammonia, Nitrite and Nitrate all always low or non existant.


Any help would be great!

Thanks
Mark

Cheers for the help! I'll monitor the angels and tank parameters closely the next few days. Relaly really cant see anyhting wrong so far. They are full of energy, bright eyes and eating well. If anything goes totally wrong this will be the first place I'll ask.