The cameras took excellent photographs in black and white, but various tricky problems prevented their use for color or, more important, for accurately measuring the intensity of the light they detected.
Compositional information is revealed by the ratio of reflected light at different wavelengths.
For one thing, the cameras did not respond in a simple, linear way to changes in intensity. In addition, the response of pixels was not uniform across the detectors. S ome portions of the pixel grid did not respond at all, producing prominent blemishes on the images. This was correctable because the spacecraft flew past Venus on its way to Mercury and took images of the relatively featureless venusian atmosphere. The blemishes are clearly visible on the photos of Venus cloud tops.