We usually view objects when they are illuminated by white light, usually sunlight or ordinary room light. White light is a mixture of all colors, in roughly equal proportions. White objects look white because they reflect back all the visible wavelengths of light that shine on them - so the light still looks white to us. Colored objects, on the other hand, reflect back only some of the wavelengths; the rest they absorb. For example, if white light shines on a red ball, the ball reflects back mostly red light, and so we see red. Most of the greens and blues that are part of white light are absorbed by the ball so we cannot see them. Likewise, a blue book is reflecting the blue part of the white light spectrum. The red and green parts are absorbed by the book.
What happens when red light shines on a red ball? It continues to reflect the red light, and so it is still red -- but a white ball would also look red in red light, because it reflects all colors. If instead we shine blue light on a red ball, it will look dark, because it does not reflect blue light. It cannot look red unless there is red light coming to it from the light source. And it cannot look blue because the red ball absorbs blue light. So when we ask what color an object is, the answer is not simple - it depends on what color light we are using to see the object.
One consequence of the fact that different colored objects absorb different wavelengths of light is that darker objects heat up faster in the sun than white ones do - because they absorb many of the different wavelengths of light energy, while white objects reflect most of the wavelengths. (More on this in the section on light energy.)
More about vision
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