Third graders are learning about how light is made up of many colors, and appears white to us. In fact, white light is comprised of all the colors of the rainbow (red-orange-yellow-green-blue-indigo-violet) and when light is passed through a medium, it can break or refract. This is why we see a rainbow of colors during a rainstorm, because visible light is "breaking" through the water droplets in the air.
Knowing that white light is made of many colors, this helps to explain why we see different colors. The visible light from the sun is white light. When it shines to the apple, the red wavelengths bounce off to our eyes, and the apple appears red. All the other wavelengths of light (orange-yellow-green-blue-indigo-violet) are absorbed. Why do we see green leaves then? The green wavelengths are reflected into our eyes, and all the other wavelengths of light are absorbed into the grass (red-orange-yellow-blue-indigo-violet). When we see the color black, all the wavelengths of light have been absorbed, and when we see the color black, all the wavelengths of light are reflected back into our eyes. How cool!