As has been mentioned up above in a link to another thread, the best technique is the acrylic paint. If its a really dark area that you can use a black pigment ink marker on then great, problem solved, but lighter colors are impossible to color match with just pens.
I went out and bought an 18 pack of acrylic paints.. super cheap. Used a sheet of acetate over the affected area and squeezed out ever so slightly the colors I thought I'd need. I was working with a blue sky. So I used a darker blue, and the white. As a painted would, I used the super fine brush to mix little blobs together, all the while working over the surface that I need to match. After just a minute of two, I was sooo close. When it dries, paint will be a bit darker so keep this in mind. I thinned out the paint with water as cause because in such small amounts it will tend to dry pretty quickly. But I was spot on in color. It might be just ever so thicker, but even regular canvas has these imperfections in the weave and the coating that it doesn't much matter. I used a really fine bruch, and even used an xacto knife to trim off some bristles when I was finally applying the paint, but used a thicker brush to mix it.
The reason I don't like the printing ink on acetate sheet method is that the color only works because off the spacing of individual microscopic dots. Once your brush mixed it, it turn to a completely different color, for me always darker. So for my blue sky, I could never get it to work. But with acrylic paint, you can match it perfectly and then its just a matter of technique in how you apply it.
