A little while ago I made the kids some rainbow rice.
I used the same method I used when dying pasta.
You will need:
Uncooked rice
Food dye
zip lock bags
a tray
paper or baking paper
I separated a 2kg bag of uncooked rice into smaller ziplock bags, added a few drops of vinegar and a few drops of colour to each. Closed them up and gently massaged the colour around to disperse evenly. It’s best to do one colour at a time – so the stuff doesn’t dry out while it waits for you to mix it through. 
After a little while left to dry it was all ready and I just had to pop it into a pretty jar. It didn’t last long though as the kids were pretty exited to play with it. 
The first activity we did {after just shoving our hands in it! It feels really nice, the kids enjoy just that sometimes!} was transferring from one container to another. This is really good practice for hand + eye coordination. It can get messy. But rice is not so hard to clean up. :o}
Noah likes to use the rice as a medium for his cars and trucks to have to get through. He buries them and uses his small diggers etc in here too. It has provided him with hours of fun.
We also hide small animals in here and try to find them. Chloe especially loves this activity. 
I store it in a large plastic container with a lid. When we want to use it, depending on what for and who is going to be playing with it – I will decant into other tubs or they will play in the large container itself.