Using the Paint Bucket on a different Layer

There is an easy workaround to use the paint bucket on another layer by following the below steps:

Step 1 - Duplicate the layer containing the line art. 

Step 2 - Hide the top layer so that you are only viewing the bottom layer.

Step 3 - Choose the paint bucket and the color that you want to use and apply it to the object that you want to fill.

Step 4 - Zoom in and select the object outline to remove the outline.

Step 5 - Unhide the layer that you hid.

This will give you a layer with your object art and a separate layer with your color.