Assemble your materials. Get a set of watercolor paints, paint brushes, paper towels for cleanup, clear glasses or jars, and some paper to paint on. As you set out the materials, ask the children what they think the activity is going to be, because asking questions is part of the scientific method. Once you put water in your glasses, you are ready to begin.
Begin by asking the children what their favorite color is and talk about how some colors are primary colors, and some are secondary. Pick a secondary color such as green and ask them what color it reminds them of. The children's answers will vary, but overall, it will remind them color is important in the everyday world.
Next, let the children work with the paint. Pick a primary color and have them dip their paint brush into the color and then into the jar. Ask them what happened to the water. Next, have them add another color. Again ask what changes they see. The new color is a secondary color.
A good scientist always collects data. If the children are old enough to read and write, have them write down: "Yellow and blue make green." If you are working with children who haven't learn to read, have them track their data by painting a streak of blue, making the plus symbol together, painting the yellow, writing an equals together, and then painting a streak of yellow with a streak of blue together. This reinforces speech to print and early math concepts as well as data collection.
After the experiment is over, you will draw conclusions. What happened to the water after it had a lot of paint in it? Was it lighter or darker? Did it matter in which order you put the two colors together when you made secondary colors? The children should notice more paint made the colors darker and less paint made them lighter and it didn't matter which color went first when they made secondary colors.