[css-color-4] add lab values to example

Author: svgeesus

css-color-4/Overview.bs

@@ -1796,12 +1796,17 @@ Device-independent Colors: Lab and LCH</h2>
 
 	Note: The Lightness axis in Lab and LCH should not be confused with the L axis in HSL.
 	For example, in HSL, the sRGB colors blue (#00F) and yellow (#FF0)
-	have the same value of L even though visually, blue is much darker.
+	have the same value of L (50%) even though visually, blue is much darker.
+	This is much clearer in Lab:
+	sRGB blue is lab(29.567% 68.298,-112.0294)
+	while
+	sRGB yellow is lab(97.607% -15.753 93.388).
 	In Lab, if two colors have the same measured L value,
 	they have identical visual lightness.
 	HSL and related polar RGB models were developed
 	in an attempt
-	to give similar usability benefits for RGB that LCH gave to Lab.
+	to give similar usability benefits for RGB that LCH gave to Lab,
+	but are significantly less accurate.