Young kids learn about the world via sensory input -- by seeing, smelling, hearing, feeling, and tasting. The idea behind sensory learning is to give your child sensory experiences that teach them about the properties of objects and substances, help them develop dexterity, and let them put words to the sensations.
Simple sensory activities you can easily conduct around the house teach your child the difference between light and heavy, big and small, soft and scratchy, wet and dry. Let her experience those variations first hand with these easy projects that you can make with materials you already have, from small toys and rice to water and coffee beans -- and without making a huge mess.
The obvious application of rainbow rice is to learn about primary colors and mixing colors. But you can also help your tot fine tune those fine motor skills as she learns to grasp those tiny grains of rice (and it's easy to sweep or vacuum up if you suffer a spill).
The Forester Clan has another application for rainbow rice -- practicing writing words in the rice with your fingers. While that's obviously advanced for toddlers, another idea is that you can draw shapes in the rice and let your child identify them. Older preschoolers can try drawing their own shapes.