Did you use pens? Try pencil or crayons - shading in colors. It still looks good. I still thinks this would be great on a gesso panel and tinted with washes of color - it's such a good retablo image.
I do use pens. What are your thoughts on water color pens? I've been wanting to give them a try. I haven't worked with crayons since grade school.
I think Owen @ Drawn to Catholicism uses watercolor pens and I like the subtle color and blending.
I haven't used watercolor pens - but I've used water color pencils. I did a quick retablo in your style for a frame I have - I'll try to get a photo up. I used a liner brush to draw and filled in with washes. It is on a piece of gessoed wood.
water color pencils, that's what I meant. Can't wait to see what you did.