Ctrl+A, you should see the entire picture selected. Go into edit and select 'Define Brush Preset'. This will turn the outline into a brush. Create a new layer and slap down the new 'brush' you created, which is your outline. It will float on top all the other layers so the lines remain true. Then you just carefully color, or you can be like me, super sloppy and erase the mess later Ahhh my old days of coloring in line art are coming back to me now and quite vividly. Have fun!