I like the look of colored gravel, to contrast with the bright colors of the betta and just add more interest to the tank. I did, however, notice that my Top Fin (Petsmart brand) gravel seems to be shedding its color! They were blue originally. Now whenever I do a water change, the water seems to be tinted a bit blue and I can see that the gravel's starting to look patchy.
Luckily, it hasn't hurt Rory at all and now that there's a filter, the water's not as blue. It's just unsettling and kind of ugly (since the gravel's all weird-looking now). I would recommend not buying Top Fin brand colored gravel.