Pigments remain chemically identical across media, yet watercolour is celebrated for transparency whilst gouache yields a flat, opaque film. What structural difference in the binder creates this divide, and what does it demonstrate about paint formulation?

B. Chalk modifies optical behaviour without a pigment swap

Chalk functions as an extender whose refractive mismatch with water and air scatters light, creating opacity without altering the pigment itself. Gouache dries purely by evaporation, not coalescence, and watercolour requires no curing stage.