Colouring guides

Straight answers about colour by number, including how the apps are built. Written to be useful whether or not you ever install this one.

• Color by number vs paint by number

  The same idea, seventy years apart. Where the terms came from, which one people mean today, and the three things the digital version genuinely changed.

• How color by number apps store the outlines

  Not as vectors and not as a flood fill. A second image where the pixel colours are region numbers, plus a twelve-pixel palette strip and a GPU shader.

• Why color by number apps fill the wrong region

  Your fingertip covers about sixty regions. What the app does with that ambiguity is a design decision, and most of them make it badly.