To put a bead on either one would require a Patridge style blade, I would think. Easy enough to do on the 640, as the blade is pinned, and easily swapped-out. The 18 woud require milling, but it could be done, by a competent smith.
I agree with Chief. Try some paint first. I, too, have issues with red and green colors. I start out by painting the blade white. Then, a layer of yellow or orange. The base layer of white really makes the color "pop". To get a precise point, in my sight picture, I draw a straight line, up the middle of the ramp, with an ultra fine point sharpie. When sighting the gun, this line appears as a fine dot, on a yellow background, in the middle of the rear sight valley. I think it works really well. When you get tired of the color, a little acetone removes everything, and doesn't hurt the bluing.
Testor's enamel works great, but so does the wife's nail polish. I also have some craft paint that seems to work well enough, for my purposes.
