When light strikes a surface, some of its energy is reflected and some is absorbed. The color we perceive is an indication of the wavelength of light that is being reflected. White light contains all the wavelengths of the visible spectrum, so when the color white is being reflected, that means all of the wavelengths are being reflected and none of them absorbed, making white the most reflective color.
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 reflectivity. The contrasting procedure is to add black to decrease the reflectivity. This is called shading.