Thanks to Lynne Chapman, the amazing UK children's books illustrator, for sharing a photoshop trick for applying to line drawings. What fun!
I found a lovely line drawing online program similar to scribbler too but different. If you have a tablet to draw with you might try it out. You can save drawings to your computer and then paint them in Painter or Photoshop or any painting program.
One can also create different coloured lines in a more hands on way with this program.