[css-color-4] define chromaticity diagram

Author: svgeesus

css-color-4/Overview.bs

@@ -206,6 +206,18 @@ Value Definitions</h3>
 	and the chromaticity is the same with both lights
 	(they are the same color, it is just brighter).
 
+	Chromaticities are additive,
+	so they accurately predict
+	the chromaticity (but not the resulting lightness)
+	of a mixture.
+	Being two-dimensional, chromaticity is easily represented
+	on a <em>chromaticity diagram</em>
+	to predict the chromaticity of a color mixture.
+	Any two colors can be mixed, and the resulting colors
+	will lie on the line joining them on the diagram.
+	Three colors form a plane, and the resulting colors
+	will lie in the triangle they form on the diagram.
+
 
 	When the measured physical characteristics
 	(such as the <abbr title="measured color, irrespective of lightness">chromaticities</abbr> of the primary colors it uses,