Hi
I am speaking from my personal experience. I have tried various colour of sands in my cory tanks, in my opinion, bright orangey colour(in my case,Sudo or Gex brand buttom sands)sands brings out the colours of the corydoras the best. White colour is one big disappointment.The corys turn out to be pale looking.It could be the reflection of the sands colour into the corys body or could the corys actually change their body colour to blend with substrate? obviously,i am no expert
With orangey or red colours,the corys colours often appears rich. I have never tried black, but did tried gray colour substrate, the results isn't too bad.
Mark