That would be true except the fish has definately changed colour. Whether it is the lights not reflecting off of the substrate as much or whether the fish feels safer, it is definately more enhanced, no two ways about it really.
Actually it's the black background: even with white sand and a black background, the fish would have looked "more enhanced" to you.
The most common reason I can think of for fish to display a lighter colour on white substrate is lack of confidence from them; some plants, caves/wood/rocks and a dark background will very often fix that.
I would argue that it is the colour of your substrate which affects the colour of the fish and that the colour of your background is irrelevant. Looking at fish, you will notice that they have a lighter underside and are darker on top. This is an evolutionary trait acting as a form of camouflage - predators looking down on the fish see the darker topside against a dark substrate, whilst those looking from below see the lighter underside against the bright sunlit waters surface. If the substrate in a natural environment was an unnaturaly bright colour, like Argos play sand for instance, the fish would be easier to spot by predators and might be quickly eaten. Hence when putting fish in an aquarium with bright substrate it causes them to feel exposed and triggers them to lighten. Fish show their colours way better when swimming over dark or natural coloured substrate, fact!
There's quite a big difference in fish colour in the OP's photos, and I'd wager if he darkened the substrate further still the fish colour would deepen even more. My own fish showed their colours a great deal more when I switched from play sand to black gravel and I have always had the same background and decor.
Sorry, that didn't sound quite how I meant it. What I was trying to say is that substrate does affect the colour of the fish (and to some extent our perception of it), while background affects our perception of the colour of the fish (and to some extent the colour of the fish).
On a profiled screen, the fish in the photos looks like the difference was caused predominantly by the latter (so it is in what our brains tell us about the colour).
[Unrelated, but this is why having a background with the same substrate makes it look like there is a huge difference to the actual fish, as people always point out after adding the background.]
