achromatic colors are colors without hue:
They does not come from the visible light spectrum.
They shows:
To create an achromatic color from a hue color, you mix at equal quantity:
They cancel each other and create a grayscale color.
Example: A greyscale of 4 colors, with the Rgb colorspace:
<div id="swatches">
<div style="background-color:rgb(10%,10%,10%)"> </div>
<div style="background-color:rgb(80%, 80%, 80%)"> </div>
<div style="background-color:rgb(95%, 95%, 95%)"> </div>
</div>