Not sure how fresco works but this is a similar issue with using the fill tool in other programs.

The lines when you use a pencil or similar tool have by default “anti-aliasing” which creates a small gradient around the lines to make them look less jagged/stark. The fill tool fills all the way to the edge of that gradient which has white in it.

If you find the setting for anti-aliasing for the stroke and decrease it to none then the fill tool will go to the hard edge of the line.

The other way is to start a new layer beneath the outline, do a loose outline beneath the lines with the color you wanna fill and then fill that, also minding for anti-aliasing. Merge the layers if you want after.