Fabrico Markers have a brush tip and a fine tip and they're colorfast after you heat set them. I use them to color quilt labels and write the info on them.
I've even used them sometimes to color "pokies" from where my tension wasn't quite right and even to tint fabrics that were a little too intense. The pastels and greys are great for this. The darker colors would be good for camouflaging little defects or stains on the fabric.
Tee Juice fine line markers might be good for this, too - they're supposed to be launder-able and lightfast. I've never tried them.