When I worked in a center, I remembered that we made homemade bath salts as a gift for moms. It was a very simple process as we used colored epsom salt, a little bit of food coloring, and some scented oil. Today, we made some sensory salt to play with using the same method.
We started with Epsom Salt. (1/2 bag for each color, but you can make as little or as much as you would like). Then we added a few drops of food coloring. (We made four different colors: Red, Yellow, Green, and Blue). I didn't have any essential oils on hand to scent it, so we used a couple of drops of cooking vanilla.
Mix it well with a spoon and be sure to let it dry before you play with it. (If you don't, your hands will turn colors from the food coloring.)