It's just been discovered more recently that since fish evolved living over darker substrates, that many species find the bright white of the light substrates stressful, which can dull their colours too. Also that it can affect their eyesight, especially with the fake looking, brilliant white sands (and we're talking tropical fish from lakes and rivers at the moment, not marine fish and marine sand). The tank lights reflect quite blindingly on those very white substrates, so more hobbyists are turning to darker and more natural substrates, trying to replicate fishes natural environments and show them at their best too.
Lighter coloured substrates aren't automatically bad though, if it's a more natural sand colour, with flecks of grey, brown, orange etc, and isn't that brilliant, fresh paint white, then it's not so bad. But also worth considering that most tropical fish are from riversand lakes, that have plants and overhanging vegetation, so bright lights and an exposed water surface can stress them out. Floating plants like frogbit or salvinia help provide that cover that helps them feel secure, and shade them from bright aquarium lights. We have to try to balance the lower lighting many fish prefer, with our own perfectly natural desire to be able to see them! And what lighting plants will need if you're going with live plants.
If you go with a darker substrate, aim for browns or very dark grey, rather than a pure black dyed one, since a pure black substrate is also unnatural, although at least not potentially blinding like the brilliant whites.
I liked the look of the very white sand substrates too, but changed my mind and got what's called black limpopo sand in the end, and really happy I did. It's a slate grey with flecks rather than a real black colour, and the fish show really well against it, makes their colours pop, as well as the plants. This is what I went for in the end;
Just try to avoid sand like this;
Looks nice to us, not so nice to the fish. Even in this photo the neons and cardinals look washed out and pale, nor the brilliant blues and reds they're known for.