This online utility allows you to quickly flatten an inked illustration. You send an image with line art and Flatton fills each white area with a random color. It then grows each area so that it bleeds under the line art, touching the other neighboring areas.
Don't worry! Those are not the final colors! They were chosen to be dull middle tones on purpose, so you can tell which areas you need to choose a color for still.
You can change the colors with the bucket fill tool in your favorite drawing program. Just place the flat colors underneath the line art and set the line art layer mode to 'multiply' so the color shows through. Duplicate the color layer so you have a backup, and start changing the colors in the color layer.
Currently only png is supported. The input file should not be larger than 20Mb and it should not have more than 80 million pixels - 8000x10000 pixels for example, which should cover an A3 at 600dpi.
I reserve the right to log images for diagnostic purposes. I try to remove those images from the server after diagnosis. Otherwise, no images are stored on the server. The entire operation is done in-memory and the resulting image is returned to your browser.
If you do worry about trusting this server, you can also use the free open-source offline version of Flatton, which is available here. You can inspect the code yourself and use this version to flatten your line art locally on your own computer.