Viking Orbiter Mars aproach pictures. The images were processed to aproximate "real color" by adjusting gamma to 2.0 and processing the channels as follows:

B (~ 440nm) = 80% violet (410nm) + 20% green (550nm)
G (~ 510nm) = 30% red (625nm) + 70% green (550nm)
R (~ 650nm) = 100% red (625nm) - 5% green (550nm)

The actual figures are approximate, but the match the visual response of the human eye (I've seen different peak wavelengths listed, but here I'm using the same as values as on my previous Viking Lander images. They seem to work and reduce excess violet or green hues in Mars images).

I'm unsure about gamma adjustment, as I don't know if the raw data was calibrated for the vidicon actual response.

Finally, I've reduced image size by 75% and applied a slight unsharp mask.