How is the hidden picture turned into cells?

Each cell takes the named color most of it shows in the picture (never a blend of two); the colors covering most of the picture stay, as many as the readers color with; a lone cell unlike all its neighbours takes theirs, so the picture has no noise. With your own pencils, each color kept then gets a pencil of its own, the colors covering most of the picture choosing first, so even a small set keeps the shape apart from its background (a blue bird does not vanish into a light blue sky).