Why a Shadow Can Have a Colored Edge
A shadow is not always a patch where all light disappears. Its edge often lies in a penumbra—the partially shaded region where an object blocks some light sources but not others. If those sources have different colors, that region can take on a tint.
Each Light Makes Its Own Shadow
Imagine a red lamp on one side of an object and a blue lamp on the other. Where the object blocks the red lamp, blue light can still reach the surface. Where it blocks the blue lamp, red light remains. Between those regions, both lights may reach the surface, blending into a different color.
The sources do not have to be separate lamps. A broad light source can be treated as many points spread across an area: the object blocks some parts of it while leaving others visible. That is why shadow edges soften instead of ending in a razor-sharp line. With colored illumination, the changing mix of unblocked light across that soft edge can produce a distinct red or blue fringe.
Why the Edge Changes Color
Move the object, the lights, or the surface, and the overlap changes. The colored border is therefore not pigment on the object or a colored outline cast by it; it is a map of which light can still reach each part of the surface. Even a small shift can move the red and blue regions, making the shadow seem to change color as it moves.
