Is it possible to only color lines and not fill in shapes? Sometimes I forget to switch my brush to gray when drawing backgrounds
You could use the Ink tool, located within the Paint Bucket tool list, to repaint lines that have been created with the Pencil tool. Such lines have a central vector line, rather than a contour one.
If you have drawn your lines with the Brush tool, you could remove the colour fills and put them on a sub Art Layer (Line Art, Colour Art, and such).
If you do not want to go through the colour fill separation, you could also head to your Colour window and right-click on the colours that make up your fills, and select the option Protect Colour.
This option prevents you from repainting any areas where that colour ID is used.
I hope this helps,