Train Flow

Sort the crowd by colour and clear the platform!

The platform is packed. The train is waiting.The crowd on that platform is a picture, and every passenger in it is one pixel. Your train loads one colour at a time.Drop in a yellow wagon and the yellow crowd rushes aboard, and that patch of the picture empties out. Fill all four wagons and the train pulls out. Another rolls in. Keep going and the crowd thins, block by block, until the platform is bare and every pass…