What you need:
Play sand. I like the bags Lowe's carries. It is very soft and fine. Home Depot's seem very rough and gritty.
Fill bowl with desired amount of sand.
Fill bowl with just enough water to cover the sand. Apply several drops of food coloring in the shade of your choosing. Apply more drops if you wish for a darker color.
Let sand sit for several hours. You can also stir the sand every once in a while to ensure the coloring is being evenly distributed.
Dump the sand onto a flat surface (I used an old cookie sheet) to dry. This could take a while, so if you are in a hurry, you can dry out sand in the oven. Heat oven to 250 degrees and put sand in with the oven door opened until it is dry.
All of this was made for a LOT less than you can purchase the colored sand for in the stores. The 50lb bag was only $3 and that will get you a lot of sand to color and play with!