Their cadmium colors are as good as anything I ever ground myself. Their cadmium red light, for example, is the best warm vermilion substitute I’ve found. It reminds me of the older, real vermilion pigments I learned to grind paint with. Their other cadmiums (red medium, yellow light and medium, and orange) are all staples on my palette. They have beautiful hues and, because of their great tinting strength, one tube will last a long time. Cheaper paints usually have a lot of filler in them, so you go through the tubes much quicker as changing a color will require much more paint.
Their cerulean is beautiful -not Old Holland beautiful, but half the price. Their cobalt blue is also good, though it seems hard to botch a cobalt blue, I’ve never used a bad one.
Not all of their colors are great. For ochres I prefer Old Holland’s golden ochre or Zecchi’s own Roman ocher. I also find the Williamsburg ultramarine totally unusable, Old Holland’s ultramarine dark is much, much better.
Williamsburg’s ivory black is the closest to hand-ground that I have found, though it is still a touch lighter. They sell a flake white too, but hand-ground lead white is really essential to good studio painting.
In 2010 Williamsburg was bought by Golden, the acrylic paint company, hopefully nothing will change.