That lander had all sorts of fiddly protrusions on it, so the multiple charts could have been to avoid self-shadowing all of them. Or they were worried about chart stability after shipping it across interplanetary space. Or the test targets were for angles for multiple cameras?
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https://en.wikipedia.org/wiki/Color_chart is even better if you want to just know about the subject.
The Viking Mars Lander had an early colour chart (three times over, for some reason) as shown in this NASA photo: https://www.msss.com/mars/pictures/viking_lander/22a158corr....
That lander had all sorts of fiddly protrusions on it, so the multiple charts could have been to avoid self-shadowing all of them. Or they were worried about chart stability after shipping it across interplanetary space. Or the test targets were for angles for multiple cameras?