because when plants are green that means they are alive. you can even give some dead leaves and explain the difference between green and brown.
have them put their hands in a bowl of cool water, and explain that water is blue. tell them that small amounts of water are very light blue, almost clear with no color, and large amounts of water, like rivers or the ocean, are very deep blue.
explain that heat, such as a fire or candle flame, or a hot stove burner, is red. red can usually be thought of as heat, or even a burn.
explain that concrete, such as on walls or sidewalks, is grey. metal 