We used colored rice as the base of our rainbow sensory bin. The rice can be made by adding 2 tsp of rubbing alcohol into a plastic baggie, along with 8 or so drops of food dye. Then add 1-2 cups of rice, close up the bag, and shake it up until all rice is colored. Let dry for at least an hour before using. Repeat this process for the different colors you want.
After the rice it was just a matter of finding different colored toys and craft supplies to incorporate into the bin to make rainbow colors. We ended up using balloons, pipe cleaners, colored craft sticks, foam cubes, mini clothes pins, toy cups, counting farm animals and spoons for them to dig around with.
From there, it was a matter of putting down a sheet on the floor (this is a must, because it’s gonna get messy) and letting the kids explore the bin. This is a great chance to teach colors, texture and shapes.