In 1969, Mariner 6 and 7 flew by Mars. The wide angle camera on each spacecraft carried red, green, and blue filters (actually two green filters). Such combinations would be used for color imagery on later missions. However, on Mariner 6 and 7, the image return was limited by data storage and transmission limitations, constraining the number of images returned and meaning that not all the data could be returned in a digital format. The filters were mainly included to better understand the photographic properties of Mars for future missions. This mission was more concerned with coverage. Hence, in most of the images taken, the color filters don't overlap, and hence color views cannot be reconstructed. However, prior to the near encounter phase, Mariner 7 took a color approach sequence in real time. Since only one in every 7 columns was transmitted, missing sections have been interpolated and the images have been stacked to improve sampling
After these images were taken, Mariner 7 began its Near Encounter phase. It took the lone closeup color mosaic of the mission, covering the Meridiani region, featured prominently in the approach images and now home to the Opportunity rover. Part of this image is made from Red-Green-Blue data, with the missing filter reconstructed for any areas with only two colors available. Areas with only one filter available are not included.
Finally, here is a narrow angle shot showing the area in the dark region with resolution of a few hundred meters. I used the wide angle image above to color it.
Processed images Copyright Ted Stryk, Raw Data Courtesy NASA/JPL