The twin Viking Orbiter spacecraft arrived at Mars during the summer of 1976. Viking Orbiter 1 entered orbit on June 19, followed by Viking Orbiter 2 on August 7. Each orbiter spent most of their operational lives in a ~300 x 33,000 km orbit, modified occasionally to change the orbital period and enable studies of the surface at different times of day. A short summary of mission phases and operations can be found here..
Viking's cameras were known as the Visual Imaging Subsystem (VIS; more info here). VIS consisted of twin-mounted 475 mm f/3.5 telescopes. Slow-scan selenium vidicon (analog TV) cameras were mounted to each telescope. Each camera returned 1204 x 1056px images, with ~11 columns on the left and right edges of the frame covered by a detector mask. In practice, this means that imaging data is 1182 x 1056 px in size, with two strips of null data. Imaging sequences usually consisted of photos taken alternatively by the A and B cameras to forming overlapping strips.
VIS was equipped with a six-color filter wheel. There were two broadband filters. One ("clear") covered the entire spectral range of the vidicon sensor from 350-700 nm. The other ("minus-blue") excluded near-UV light and extended from 480-700 nm. Four narrowband filters split up the visible spectrum. Band centers of these filters were at 410 nm ("violet"), 440 nm ("blue"), 550 nm ("green"), and 625 nm ("red").
Viking Orbiter data is most easily searched and downloaded at the PDS Imaging Node Planetary Image Atlas. This data is available in *.IMG format and can be converted with IMG2PNG with no issue. File names take the form of "fxxxa" - f being a prefix, xxx the last three numerals in the orbit number (001-999), and the last letter being the spacecraft ID. ('a' and 'b' are Orbiters 1 and 2, respectively; 'c' and 'd' are the Orbiters 1 and 2 prior to orbital insertion, respectively; 's' is Orbiter 1 after orbit 999).
Searching for color sets at the Imaging Node is easiest by selecting the "filter" option. The green filter was almost exclusively used only during the aquisition of color photo sets, and in most cases images acquired using the violet filter were accompanied by images taken with the red filter. Red filter images were used to track surface albedo over time, and are commonly associated with single-color observations.
NASA Special Publication 441, "Viking Orbiter Views of Mars" is a broad overview of the Viking mission and collects 80s-era processed versions of the best image mosaics. Unfortunately this guide is not totally useful for identifying the data sets used in the production of these mosaics - although some mosaics list the files used in their production, others are listed by mosaic number or JPL Image Processing Lab catalog number.
A more complete listing of imaging mosaics and observation codes can be found in the Viking Mosaic Catalogs. Volume 1 contains mosaics prior to ~1980. Volume 2 contains observations used to produce quadrangle maps of the Martian surface and mosaics produced after ~1980.