Here's a little tutorial on how to fill a lineart without any ugly pixels sticking out.
Step 1. Go to lineart layer. Press Ctrl+A to select everything.
Step 2. Choose Magic Wand tool and deselect all spaces outside lineart.
Step 3. Go to Select>Modify>Contract. Contract by 1 pixels (or more if your lineart is thick).
Step 4. Use Lasso tool to deselect small spaces Magic Wand was unable to reach.
[b]Step 5.[/b] Go back to colour layer (with selection still on). Grab some basic colour and fill in selection.
Step 6. Lock the layer for transparent pixels and have fun colouring.

I hope this was helpful.