Saturation
How vivid a colour is, from full strength down to grey.
Also: purity
A fully saturated colour is vivid, with no grey in it. As saturation drops, the colour looks washed out, and at zero it is a grey. Photo editing sliders work this way: push saturation up and the picture becomes bolder, pull it down and it turns black and white. In design software saturation is given as a percentage. Highly saturated colours draw the eye and tire it quickly, which is why they are often kept for small areas. Bright light and distance both tend to lower the saturation we see.