Color Charts for Mars
>>>A grey card seemed to work well for a long time….
That’s because film has the color science baked in. If you print the gray card as neutral then everything else falls in line (depending on the processing, of course, and even then I don’t think you could seriously skew the color short of cross processing or doing something very strange).
Digital is completely different. There’s a lot of math going on under the hood and it’s not fixed. There’s always a matrix operation in there somewhere and that affects color only, NOT NEUTRALS.
I’ve told this story before but it’s always relevant:
Years ago my regular video engineer and I met with an engineer at Bexel to get a quick training on this new thing called “HD.” I had a specific question about the Sony F900 user matrix and the engineer made the point that I shouldn’t touch it lightly. He placed a gray card in the middle of the set, zoomed in on it, and put random numbers into the user matrix. Nothing changed. Then he zoomed out. Every color on the set was whacked.
The Sony F900, like most broadcast-style cameras, had four separate matrix operations happening in a chain: at the bottom you had the OHB (optical head block) matrix meant to correct for color variances in prism blocks, then the matrix that defines the color space (Rec 709, NTSC, EBU), then the user matrix (to allow the user to adjust overall color as desired), and then a vector matrix (to allow the user to adjust individual hues within a set range). None of these affect white, gray or black.
Modern cine cameras don’t have this exact setup but there’s always at least one matrix operation happening somewhere in the imaging chain.
As the color science in digital is extremely malleable and exists primarily in software, it’s very easy to get lost. A gray card will help you balance the red, green and blue gains but that’s a very basic operation (white balance) and is usually only the first step in creating viable color. The spectral response curves of the CFA filters overlap and those signals must be added to and subtracted from each other to create basic color. That’s what a matrix does. It’s a fundamental operation that affects color in every way but it won’t touch white, black or gray.
To sum up: if you’re just trying to correct an overall color cast, a white or gray reference is just fine. If something is skewed in the camera matrix (or there’s a LUT imposed and it’s the wrong LUT) you can’t save yourself unless you have known color references to work from.
Generally the least you need for terrestrial efforts is white/black/gray, red/green/blue, yellow/cyan/magenta, and it helps to have a flesh tone reference.
I’m assuming the image processing for off-world image capture happens here on Earth. If color looks a bit strange there’s no way to know if that’s due to the subject or the image processing. If you have color references on location then you can figure this out because you can solve for the color references. A neutral reference is not enough.