White, black and gray are called achromatic colors, which means "a color without hue".

White is an achromatic color that is a mixture of the frequencies of all the colors of the visible spectrum. A neutral or achromatic color means literally that a color has no color. By the way, it is one of the most common colors in nature that is the color of sunlight, snow, milk, chalk, limestone and other common minerals. In many cultures white represents or signifies purity, innocence, and light, and is the symbolic opposite of black, or darkness. According to surveys in the US and Europe, white is the color most often associated with honesty, pureness, perfection, the good, cleanliness, neutrality, exactitude and something new.

More Info: en.wikipedia.org