Welcome to the Forum! There are different opinions on the subject, though I personally agree with those who say a darker substrate and darker background will generally bring out more coloration in Flowerhorns. That said a dark background and something like natural river gravel (smooth is best) as Thomas suggests will work well too. In fact my FH has done quite well color-wise with blue or black backgrounds and (for first year) just bare bottom with white foam sheet under it. This leads me to personally believe that background color might be more important than substrate color, but just my own little theory.
Bear in mind too that good tank size, proper/clean water parameters and diet will have even more affect on your Flowerhorns's coloration and happiness.
Good luck and show us some pix later