Every shade of gray is a mixture of black and white. Darker shades have more black, and lighter shades more white. Some shades, such as cool gray and slate gray, include a small amount of some other hue, but true grays are defined as achromatic colors between white and black.
Calling gray, white or black achromatic colors is technically inaccurate, since the word "achromatic" literally means "without color." A black object absorbs all of the visible colors of light, so the reflected light has no color. A white object reflects all of the visible colors, blending them into apparently colorless light. All of the shades of gray exist somewhere on the gradient between white and black, and so gray is not technically a color either.