I use the Pitt's Pens a lot...they are great for covering a little blob of thread on the back of your quilt--it just disappears from sight, for slightly changing the color on an embroidery that stands out wrong, for coloring along the edges of a raw-edge applique if made from a printed fabric so you can't see the lighter colored edge, for adding window pane frames on your house, for coloring in the stones on your stone path for your landscape quilt (see
http://blog.bjfabricartist.com/2013/04/03/landscape-quilting--create-a-stone-pathway-9.aspx), for darkening the beard on your St. Joseph applique, for adding highlights and low-lights to your appliques...in short, I don't know what I would do without them. I have found that once I heat set them, they are very permanent. But you want to try them out first on your fabric, because the colors are somewhat influenced by what the base fabric or thread color is.