Gray-Scott model: ∂u/∂t = Du∇²u − uv² + f(1−u), ∂v/∂t = Dv∇²v + uv² − (f+k)v. U = activator, V = inhibitor. Pattern type depends sensitively on (f,k). Click canvas to add V. These equations model skin pigmentation, coral growth, and chemical oscillators.