What are ACFs?
Do you mean African Clawed Frogs?
Anyway, many aquarium stores sell colored sand that can be used in fish (and frog) tanks.
Depending on what colors you want, you can also find various sand types at construction materials yards that sell stone, gravel, and sand.
Some craft stores also sell sand in bright colors, but unless it's specifically labeled for aquarium use, I would first test it by keeping it in water for a few weeks (to see if the color comes out in your water) before using it in a tank.