Pour 10 drops of liquid food coloring into the bag of oats, along with one teaspoon of white vinegar. Seal the bag and shake the oats around to spread out the food coloring more evenly.
Open the bag and add 5-10 more drops of food coloring and ½ teaspoon of white vinegar. Seal the bag and shake the oats around. Repeat this process one more time with 5-10 drops of food coloring and an additional ½ teaspoon of vinegar.
If your oats are still not colorful enough, add a few more drops of food coloring and shake the oats to spread the coloring throughout. Repeat as needed, adding food coloring but no vinegar, until you are satisfied with the color of the oats. (Darker colors may require more food coloring.)
Pour the colored oats onto a baking sheet to dry. Once dry, the oats are ready to play with!