We present a real-time method to capture and simulate the dynamic behavior of watercolor painting. We develop a physically accurate, grid-based, real-time fluid simulation based on a reparameterized Thin Film model. The equations are rewritten so as to create principled parameters that finely control the length, thickness, and frequency of dripping. Our close connection with physics allows both theoretical and experimental validation of our method. The resulting system can reproduce dripping, fluid-air interface, and pigment advection and diffusion, all controllable by the user in real-time. Our experiments show that artists can use our system to create interesting and varied digital paintings.