Moving from one colour block to the next executes a command based on the hue change (6-step cycle) and the lightness change (3-step cycle, light → normal → dark → light), exactly as in Piet — the command depends on the difference between steps, not on either colour alone.

The command executed when exiting a block of size N and entering the next, for each hue change and lightness change.

A paint program for pgmpiet, the grayscale derivative of Piet. Palette: black (33), white (255), and 18 grayscale steps (112…247) in canonical hue-major order. The Run panel embeds a full interpreter so you can test programs without leaving the editor.