As of now we do not have the option to color-in existing images in. You do however have the capability to change the Doodle outlines by clicking on the gear icon after you select an asset.
You can also simulate coloring-in any asset that you import into the app. To replicate an image being colored, import an existing colored image and add draw paths to it. Put in the draw paths in such a way that the outlines are drawn first and the inner areas after. This will animate as if the colors are being shaded in.
Please note: We will have the option of colored images in the near future. Stay tuned!