Grocery store dyes are hot-water dyes. The secret to spectacular tie-die (and batik) is cold-water Procion Dyes. These come in scores of brilliant colors, and can be found in larger art supply stores. To start with you’ll only need the smallest size they sell, an ounce or two of dry powder, plenty for maybe a hundred shirts. Dissolve the powder in clean empty squirt bottles and you are ready to go after you soak the designated clothes in Arm and Hammer Washing Soda (sodium carbonate, for a fixative). In our experience you’ll want to maximize the concentration of the liquid colors to keep the end result brilliant.