Similar to coloring eggs, use 1-2 Tbsp of vinegar and some food coloring. My food coloring was the gel kind, so I shook it up in the vinegar until dissolved, before adding 1 1/2 cups of rice. Then shake to coat the rice – we used a canning jar and plastic lid.
I also hid some small toys in the rice while Elaine was closing her eyes. Then she had to dig through the rice to find the treasures. We had too much fun, and she has asked to play with the rice on several occasions since!