If you're just using gravel then it doesn't matter that much, it mainly depends on what you like best. As Kipper said certain fish may show less certain colouration against or not contrast very well against certain coloured gravel, it's a defence mechanism from above predation. I had Red cherry shrimp in a tank with cat litter which is reddish in colouration and the shrimp just looked brown in colour, they looked a lot better in my other tank with brown gravel.
Word to the wise, if you're swapping them over in one day if you can I would do it gradually over a number of weeks using water from the original tank and if possible some of the media out of the filter. I know this might not be possible if the tank is going in the same position. Failing that use as much stuff as you can from the original and plenty of good sized water changes to start with.