When light strikes a surface, some of its energy is reflected and some is absorbed. The color a person perceives indicates the wavelength of light being reflected. White light contains all the wavelengths of the visible spectrum, so when the color white is being reflected, that means all wavelengths are being reflected and none of them absorbed, making white the most reflective color.
TL;DR (Too Long; Didn't Read)
As a form of energy called electromagnetic radiation, light travels in waves with some of its colors having longer wavelengths than others. The visible light humans see as the color white consists of a rainbow of colors in the electromagnetic spectrum that range from blue to red, with yellow, orange, green and multiple variations sandwiched between them, as in a rainbow after a storm. Blue and violet have shorter wavelengths and higher energy, and, at the opposite end of the spectrum, red wavelengths are longer, but have lower energy.
From Total to Zero Reflectivity
If the color of a surface is anything other than white, it means that it absorbs light of some wavelengths. For example, a surface that appears red absorbs yellow, green, blue and violet light, while reflecting red light. A surface that appears green absorbs all colors except green. White light is a combination of all colors -- as is apparent when you shine a white light through a prism -- so anything that appears white reflects all wavelengths of light. Black is the least reflective color, it's the color of a surface that absorbs all light.
Tints and Shades
If a surface isn't white, then the closer its color is to white, the more light it reflects. Pastel and off-white colors reflect more light than deep tones. Adding white to a color is called tinting the color, and it increases the color's reflectivity. The contrasting procedure is to add black to decrease the reflectivity. This is called shading.
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Different Colors in Different Lights
An object that is white, would look red in a red-colored light because white contains all colors. But if a blue light were shined on a red ball, the color on the ball would be very dark, because the red color only contains red, not blue, so it absorbs the blue light instead of reflecting it. The color of an object depends on the light cast upon it. The only way to know the color of an object is to put it into sunlight or white light.
Darker colored objects heat up faster in the sun than light colored ones, which is why running across asphalt in bare feet can feel much hotter than walking across light-colored concrete. The reason is that darker colors absorb more of the different wavelengths of light energy, while white or light-colored objects reflect the light of most wavelengths.