I've learned that before gluing my tiles together, I not only edge the perimeter of the tile, but also around the tab flaps and underneath the corners. On my original tiles, I didn't glue the sides so they were vertical - I glued them so they slanted in a little. This caused some overlap to show the underside of the cardstock. Coloring under the cardstock a little prevents this from happening again.
Here's another edging shot: