Garden Mama made this by having her kids paint paper with liquid watercolors then cutting the paper into hearts after it had dried.
Liquid watercolors are bright, colorful and very economical. When I was teaching this was always my paint of choice for the kids. I could buy the 3 primary colors plus black and brown and be set for several years. You can even add water to dilute it a little and still get brilliant colors.
If you need liquid watercolors to do this project you can get this - 1-Oz. Color Splash! Liquid Watercolors (Pack of 6) from Amazon for only $7.99. Use Amazon Mom and get FREE shipping.