The Matrix Gallery is a section of the Wadsworth Atheneum which has provocative shows for the region it serves, although its choices are fairly New York establishment. What was interesting about this particular show was the presentation of two artists, and two artists whose art is obviously so different. John Baldessari’s photographs were chosen to point up his interest in color. Some of the newer sequences were shown at the Whitney Biennial; others were more than five years old. Color is a feature of his work that most people have not discussed, except to call it “sumptuous,” which doesn’t say