Colorize B&W marker drawings with the paint bucket

Make sure all your shapes are “closed” when you do your marker sketch.

Scan the drawing, and adjust levels.

In Photoshop, position your scanned drawing on the lowest layer.

Make color layers above the drawing, and set each one to MULTIPLY.

Use the bucket tool set this way:

Because of the settings above, the fill bucket will detect the areas on the drawing layer below.

Due to the multiple setting, the black lines of the marker will show through without being diminished by the fill, which slightly overlaps the lines.