Brown: In the CMYK color model used in printing and painting, brown is usually made by combining the colors orange and black.
Black: To be honest, this could serve as a border for each part of the rainbow spectrum.
White: a color without hue at one extreme end of the scale of grays, opposite to black. A white surface reflects light of all hues completely and diffusely.
Gray: It is a neutral or achromatic color, meaning literally that it is "without color", because it can be composed of black and white. It is the color of a cloud-covered sky, of ash and of lead.