[css-color-4] Used a more precise definition of achromatic colors with missing hues (sufficiently close to the central axis)

Author: svgeesus

css-color-4/Overview.bs

@@ -1074,7 +1074,8 @@ and prophoto-rgb green is 141.04 degrees
 (because these are all different shades of green).
 
 <<hue>> components are the most common components to become [=powerless=];
-any achromatic color will have a [=powerless=] hue component.
+any color sufficiently close to the central achromatic axis 
+will have a [=powerless=] hue component.
 
 
 <h3 id="missing">