The color of an object depends upon which wavelengths of light it reflects back to your eyes. If the object changes color, then your eyes are seeing one wavelength of light and then a different wavelength of light.
So what causes the wavelengths of light reflected by an object to change? It all depends upon the object and the particular circumstances under which it changes colors. As we look closer at a few examples, you'll see that different objects can change colors for different reasons.
Have you ever played with a mood ring? If so, you know that it can change colors. Manufacturers claim mood rings change color based upon your moods, but science says it all has to do with temperature!
Mood ring “stones" are usually hollow pieces of glass or quartz that are filled with liquid crystals with special thermochromic properties. That means changes in temperature cause the liquid crystals to change shape, thereby reflecting different colors of light and causing the color of the “stone" to change.
When it comes to the leaves on the trees, though, different processes are at work. Leaves contain many substances that reflect different colors of light. During the summer, the most prevalent substance in leaves is chlorophyll, which reflects green light. That's why leaves appear green most of the time.
As autumn approaches, however, lower levels of sunlight result in lower levels of chlorophyll in leaves. With lower levels of chlorophyll in the leaves, other colors that have been there all along, such as yellow, red, and orange, begin to shine through.
Chameleons that change colors do so in a way that's different from either mood rings or leaves. Unlike mood rings, chameleons do change colors to reflect their moods. Although many people believe chameleons change colors to camouflage themselves, scientists believe that's only a secondary benefit to the social signals they send to other chameleons.
To change colors, chameleons manipulate specialized cells called chromatophores. These cells contain different colors of pigment. When chameleons want to convey a particular mood or message to others, their brains send signals to their chromatophores. Their chromatophores then move pigments around to reflect different wavelengths of light. In this way, they change their colors to reflect their mood!
As you can see, changing colors can be accomplished in many ways. How and why a particular object changes colors will depend entirely on the object and the systems involved. The bottom line, though, is that changing colors means that different wavelengths of light are being reflected back to your eyes.