I took a photo of an orange lit by a single light source, then measured 10 color samples from light to shadow using the "Color Analyzer App" we developed. The three numbers on each color sample are its hue, chroma, and value. On the color wheel, we can see: (1) in the light area 1 to 5, the hue shifted from yellow to red orange. This hue shift is often referred to as the "prismatic color shift". (2) in the shadow area 6 to 10, the hue does not change. On the "value vs. chroma" diagram, we can see the variation of chroma and value is strongly correlated by following two straight lines. These two linear relations of chroma and value governs the color integrity of a 3D solid object. We may refer to it as the "saturation constancy" rule. If you violate this rule when you paint, you will have muddy, chalky, or garish color problems. Most advanced artists know the effects intuitively, but I want to express the rule with an unambiguous scientific statement. You may have heard the saying: "rules are made to be broken", but this one won't.
Thank you for viewing my art,
Qiang
