1. A method, comprising: Identifying, using one or more processing devices, a location in a display screen where a user action simulates a brushstroke; determining, based on the location of the simulated brushstroke, a geodesic distance between a first pixel in the location and a second pixel outside of the location in the display screen; determining a contribution of a color associated with the simulated brushstroke to the second pixel based on a color diffusion relationship, the color diffusion relationship comprising (i) a color diffusion component defining, based on the geodesic distance, an effect of the color of the simulated brushstroke on a color of the second pixel and (ii) a time component providing, based on the geodesic distance, a delay between when the simulated brushstroke is identified and when the color associated with the simulated brushstroke affects the color of the second pixel; and displaying the color in the second pixel based at least in part on the color diffusion relationship.