I know what he means 8)
I've noticed that fish change their colour and behaviourd depending on their surroundings. For example, I had to move my BN's out of their tank (with dark grey gravel) and into a tank with light coloured sand. They changed their pattern to be more blotchy and lighter, they also seemed less active and wanting to hide more. The light reflection of the lighter coloured substrate can be bright for many fish and hence they change their colours to blend in.
Now, how to change it. This has been a popular topic recently, try reading through some of those threads. The general method of doing it is removing fish into another tank or container with the heater, filter and tank water too. Remove old substrate, place new substrate, replace with old tank water, add filter, heater and fish.
HTH
(Argh! Semi-nija'd! :oops: )