[css-color-4] add cmyk round-trip figure

Author: Chris Lilley

css-color-4/Overview.bs

@@ -2598,6 +2598,24 @@ Also, procedures that rely on knowing the color
 (anti-aliasing, compositing, using the color in a gradient, etc)
 can proceed as normal.
 
+<figure>
+	<img src="images/macbeth-roundtrip.svg" style="width: 60vmin">
+	<figcaption>
+		A color checker, used for ensuring color fidelity
+		in the print and photographic inductries.
+		Averaged measured Lab values are available for each patch.
+		The rectangles show the Lab values, converted to sRGB.
+		The circles, which are barely visible, show the Lab values,
+		passed through an ICC profile to convert them to CMYK.
+		The CMYK values are then passed through the same ICC profile in reverse,
+		to yield new Lab values. These are then converted to sRGB for display.
+		The one patch with a more visible circle (third row, first patch)
+		is because the color is slightly outside the gamut of the
+		FOGRA55 CMYK space used.
+	</figcaption>
+</figure>
+
+
 <div class="example">
 	This example is using offset printing to ISO 12647-2:2004
 	using the CGATS/SWOP TR005 2007 characterisation data