I made some Kool-Aide dyed rice for a play date we hosted.
I bought 15 pounds of rice at Kroger on sale. I put 2.5 pounds of rice in a plastic bag with 1/3 cup of white vinegar and 2-3 packets of Kool-Aid. Other blogs measured their Kool-Aid but I like to live on the edge. I shook the bags until the color had gone throughout the rice and then let it dry on cookie sheets overnight.
I thought that it looked pretty laid out in bands, but of course the rice isn't for looking, it's for playing!
Though on the subject of mess, I've heard horror stories of rice breaking vacuum cleaners so I swept most of it up and used the vacuum only for the last bits. Most of it I used the hose attachment and only rolled up the last pieces.