The two photos shown here in sequence were both taken with Opportunity’s Pancam and with the same filters, but nine days apart.
Since each individual image is the sum of three photos taken at slightly different times through three different filters in the three basic RGB colors, I chose to refer to the photo taken with the green filter for time references.
Starting from this assumption, the local solar time has varied by 1h 43min and 36sec “Martian” corresponding to a shift of the illumination source (the Sun) of 25.9 °. I think it is rather unlikely that such a small shift of the illumination source (the Sun) can be at the basis of such an evident chromatic variation!
Note also that the color variations affect only the green-blue areas and not the reddish areas.
It should be added that the two photos have been taken with slightly different exposure times, but this may have affected only the saturation of the color and not the hue of the color itself (chrominance).
I conclude by pointing out that such color variations are difficult to justify with arguments of a simply physical-chemical nature and that I find much more likely a variation in the growth of lichens.
Far from being proof, it is certainly a very good clue!
HD animated GIF: https://image.ibb.co/nHtdrc/Oppy_Sol5042e5051_X2_Date.gif
For more information about the individual images:
Opportunity Sol 5042, 31 March 2018 14:40:16 LTST (green) https://www.facebook.com/PianetaMarte.MdM/photos/a.1625578140858706.1073741828.1612043482212172/1659829317433588/
Opportunity Sol 5051, April 9, 2018 12:56:40 LTST (green) https://www.facebook.com/PianetaMarte.MdM/photos/a.1625578140858706.1073741828.1612043482212172/1668172363265950/
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