At such a great distance from the Sun, with so little light, and seeing a crescent (and therefore dimly lit) phase, it's astonishing that Voyager was able to capture any photos at all. Some of its photos were sharper than others. This particular image was composed of three photos captured through clear, blue, and violet filters because they happened to be relatively sharp. Channel mixing was used in order to approximate true color somewhat more closely, and the final color adjusted subjectively to make Neptune and Triton appear their expected colors. So the final color is perhaps more "artistic" than "real," but the presence of clouds on Neptune reflects reality.