I purchased some Fabrico markers for highlighting and shading quilts a few months ago. One of my friends uses them a lot on her quilts, and since she has won so many top awards, I thought that I should learn how to use them too. Fabrico markers are a product by Tsukineko, which makes other products that are used by art quilters and paper crafters. I usually use Derwent Inktense pencils with fabric medium to add depth to my quilts, and have written a blog or two about how to use them. The Inktense pencils are really easy to use and they don't bleed when you use fabric medium. These markers also do not bleed.
I made a little butterfly quilt and after appliqueing down the pieces to the painted background, I thought that it looked totally flat and fake. I wasn't trying to make a realistic butterfly, per se, but I wanted to make it look less like a cartoon character. I used the markers and found them easy to apply, but I had to go over the area a few times to be able to see the shading.
Here is the picture of the little quilt before and after the shading, but before the quilting. It's faint for sure. You can see it more on the body than on the wings.
I hope you try new products too.