[css-color-5] Improve the fake-greyscaling calc example, fix w3c#6857

Author: Chris Lilley

css-color-5/Overview.bs

@@ -941,12 +941,22 @@ so it has meaningful values for the channels.
 
 	Using this,
 	''red'' would become ''rgb(30% 30% 30%)'',
-	''green'' would become ''rgb(59% 59% 59%)'',
+	''lime'' would become ''rgb(59% 59% 59%)'',
 	and ''blue'' would become ''rgb(11% 11% 11%)''.
 	A more moderate color, like ''darkolivegreen'',
 	which has RGB values ''rgb(85 107 47)'',
 	would become approximately ''rgb(37% 37% 37%)''.
 
+	(Rough because firstly,
+	although this looks like a luminance calculation,
+	the red green and blue values
+	are manipulated in gamma-encoded space
+	rather than linear-light;
+	secondly,
+	the weighting factors are those
+	for the obsolete NTSC color space,
+	not sRGB.)
+
 	(Note, though, that an easier and more accurate way to grayscale a color
 	is to use the ''lch()'' function,
 	as that color space is more accurate to human perception: