Isaac Newton observed in the 1600s that if a narrow beam of light is passed through a prism, it emerges from the other side in colored bands of light (Pink Floyd fans can cue up an image of the Dark Side of the Moon album cover), suggesting that “white” light is actually composed of light of various colors. The colors that Newton identified were red, orange, yellow, green, blue, indigo and violet, sometimes written as ROYGBIV; these are of course the colors of a rainbow, which is formed by light refracting through water droplets, rather than a prism like Newton used.
When a scene is illuminated by direct (white) sunlight during midday, objects in the scene appear to us “in color.” What is happening is that each object in the scene absorbs some colors of light and reflects others. What we see as the color of an object is actually the color of the light reflected by that object. Although we think of the reflected colors as the “true” colors of the objects, careful observation will reveal that these colors can change, depending on the color of the light striking the objects.
For a striking example of this, find the white balance settings on your digital camera and set to “off.” Then, on a clear day, photograph something outside that is familiar and brightly colored after the sun has gone down. (If you live in a location where the air is not particularly clear this won’t work well.) You will observe that everything in the photograph has a blueish tint that you likely did not notice when you looked at the scene. This tint is due to the fact that the scene is primarily illuminated by blue light from the sky, rather than white light of midday. Some light of the “true” color of an object is reflecting off of it, but so is a significant amount of blue light from the sky. You did not notice the blueish tint because our brains correct for the blue light without our conscious recognition of this, but the camera records the actual color of light striking the sensor (that records the photograph). When you set your camera’s white balance control to auto, it makes roughly the same correction that our brains do.
As photographers, we can leverage this effect if we understand it. An example is the photograph below, of the Deer Creek Narrows, in the Grand Canyon of the Colorado River. Although the rocks on the left appear to have an orange color and the ones on the right a purplish-brown cast, both sides are actually the same rock formation. To understand what is going on, one must understand the geomorphology of the location. The small canyon photographed is in the bottom of a somewhat wider canyon. The photograph was made before direct sunlight entered the small canyon, but when it was striking the right upper wall of the larger canyon, which consisted of yellowish-orange rock. (Remember that this simply means that the rock reflected yellow and orange light and absorbed other colors.) The reflected light was then striking the left side of the smaller canyon in the photograph, enhancing the “natural” yellowish-orange of that rock. The reflected light, however, was not striking the right side of the canyon in the photograph. That side of the small canyon was illuminated by blue light from the clear overhead sky, resulting in the purplish-brown color. Because the two sides of the canyon are illuminated by different colors of light, the same rock appears differently colored to us!