well der is another method..

Create a new layer above the LineArt.

Set the NEw Layer to "Multiply" . Color it wif 100% Opacity. U still can see ur Outline clearly. This way, U are not allow to use Fill Bucket. But u can Paint above the Outline easily clear.

i dun recommend FIllin color..sometimes it will haf jagged thing around the edges.. Best is filling or paint a little to get smooth coloring result.