I think this is finished. If you'll compare the finished portrait with the last stage of the grisaille underpainting, you should notice a good deal of additional modeling, particularly on the face.
Any time you are adding some visual element from your imagination, as the color has been added here, you are basically operating from experience, example and what looks right. I kept the color pretty subtle, but didn't want to leave the image in it's tonal state. I wanted to warm up old Gilbert and make him appear more alive.
This is something of a gamble, but worth it. I showed the grisailles underpainting at our local art group, Artists of Northwest Arkansas. When I mentioned that I next planned to glaze in some color, I got a few comments along the lines of, "Are you sure you want to do that? I like the sepia look.". People wanted me to leave it be.
The tricky bit is to make sure that everything you add to a painting (any painting) improves it. Things don't always turn out that way, but that's the idea. I don't lay a brush to my painting unless I know what I want to accomplish with it. I try hard never, ever to simply thrash, - to move paint around and hope something great happens. If a painting is not coming together the way I thought, the best thing, I've found, is to walk away. Put it aside until it speaks to me again and tells me what it needs. Nine times out of ten it will become obvious with time.
Anyway, I think the color improved this image, at least it looks that way in person. This is a mere digital snapshot. I'll post a professional scan soon.