I believe there are times to plan your art with a pencil, and there are times you just need to jump right in and draw with a brush. You can’t go wrong with these rows of delicate hearts, even if they do get crooked and lopsided. In fact, I think they may get even better.
Start with real watercolor paper, and Educational Mixable Crayola Watercolor Mixing Sets. They really are great and allow all the color variety you see here. With a small brush, cup of water and plate for mixing, challenge your students to paint rows of even hearts, mixing new colors for each one. The set comes with white and black and brilliant blues and violets so many combinations are possible.