Only the outline drawing is kept (on your device, with your picture). The photo itself isn't stored.
Pick a page and tap any area to fill it with the colour you've chosen. The fill stops at the black lines, and small breaks in a line (common on photo pages) are closed first so the colour doesn't leak into the next area; set "Close gaps" higher if it still does. The brush, pencil and eraser paint underneath the lines, so the outlines always stay crisp, and "Stay inside" keeps a brush stroke within the area where it started.
In your browser's storage on this device (IndexedDB), saved a moment after each change. Nothing is uploaded and there's no account. Clearing your browser's site data deletes them, so use Export to keep a copy as a file, and Import to bring it back or move it to another device. Undo history lasts until you close the page.
The photo is turned into outlines in your browser with a difference-of-Gaussians edge filter: it finds where brightness changes sharply and draws a line there, then removes specks. "Drawing or scan" mode keeps dark pen strokes and drops the paper instead. Simple, well-lit photos with a clear subject work best; busy backgrounds give busy pages. The photo is never sent anywhere and only the outline drawing is saved.
Print a blank page from any page's own page (Mandalas, Patterns, Animals, Scenes), or print your coloured picture from the editor's menu. Every page fits US Letter and A4.
All the pages are drawn for this site: the mandalas and patterns are generated by our own code and the animals and scenes are simple drawings made from basic shapes. They're free for personal and classroom use.
F fill, B brush, P pencil, E eraser, I pick a colour, H move, [ ] size, Ctrl+Z undo, Ctrl+Shift+Z redo, + - zoom, 0 fit. With the picture focused, the arrow keys move a cursor (Shift for bigger steps) and Enter or Space uses the tool there.