First of all, before we can talk about why things have color, we need to talk about what color is. Color is a property of light. Light can be red, green, blue or any color in between. Light can even be colors that people can't see. In fact, your radio and TV pick up light of a certain color and the x-ray machine at the dentist shoots light right through you. Radio waves and x-rays are colors of light that you can't see but are still there.

Light, you see, is a wave. When I say wave, I don't mean a wave at the beach crashing on the shore. Instead, think of throwing a pebble into a lake. When you do that, you see ripples moving away from the spot that the pebble hit the water. Those ripples are a wave. Now, the ripples have a particular height and they are a certain distance away from each other. Light acts just like it is a wave on the water. It acts like it has ripples. Tall ripples means that the light is bright and short ripples means the light is very weak. Color is how your eyes sees the distance between the ripples. The farther apart the ripples of light, the more red the light appears. The closer together the ripples are, the more blue the light appears. If the ripples are very, very far apart, you can't see them anymore. That is the sort of light, though, that your radio can pick up. If the ripples are very, very close together, you can't see them either. That is the kind of light that an x-ray machine puts out.

Now, light from the sun comes with all sorts of colors mixed up together. That sort of light appears white. So when you look at something white, that object is reflecting all the different colors of light from the sun (or a light bulb). A red apple reflects more red light than any other kind of light. A green plant reflects more green light than any other kind of light. And things that don't reflect light at all can either absorb all the light, which makes them black, or transmit all the light which makes them clear like glass.

Now, some molecules can have color. An example is chlorophyll, which is the molecule that gives plants their green color. Many molecules can be made to emit light of certain colors also, depending on what kind of molecule it is. This is called a spectrum and by studying the colors that are emitted by stars, for example, scientists can tell what kinds of atoms are in the star. Most molecules don't seem to reflect light that we can see, and so I would say they are colorless. Most things do not have the colors they have because they are made of molecules that reflect or emit light of a certain color. The structure of the object you are looking at is another thing that can affect color. Things that might affect what colors of light are reflected are the distance between molecules, or how the molecules are organized. This is why snow can be white while ice is mostly clear even though both are made out of the same molecule: water. There are, in fact, a lot of ways for things to get color.

Since most molecules are smaller than the wavelength of visual light, individual molecules would not have a color. They do not absorb, reflect, or transmit light in the same way that bulk substances do.