For this I generally use 'whatever paint is the right colour and texture' (same philosophy as for thread!) - this time I have used Jacquard Lumière in pewter mixed with a little black artists acrylic to tone it down a bit, plus a few (very few) drops of water to help it flow. A small to medium-sized round brush with fairly firm synthetic bristles is ideal, and then all you have to do is take a deep breath and go for it! I tend to mix the paint as I go for a slightly random feel, and paint up to the stitched line, but not touching it. And you can see form the picture that it really does make a difference to the finished look of the piece. These paints would normally be ironed to set, but as the quilt is a wall hanging that will never be washed, I am happy to let them air-cure.