A gray target is what you want to determine filter factors. Or course you can use something like a MacBeth Color Checker chart, which has an excellent gray scale on it as well as the color patches.
Here's part of the problem : different films see color values differently than we do; and colors in nature often reflect wavelength combinations we don't intuitively recognize; and colors in artificial charts don't behave like those in nature.
Then there's the problem of meter spectral sensitivity itself. So unless one wants to make a very complicated exercise out of all this, it is best to rely on a single common denominator neutral standard when doing filter factor tests.
And you can't assume the only use for a contrast filter involves only attenuating something on the opposite side of the color wheel - the antagonistic color. These filters are applicable for lightening similar colors too, or more mildly moderating in between hues.
And like I just implied, in nature what appears to be an opposite to our eyes might be something quite different to film. Use a red filter to darken green leaves and the result will be much lighter than you think, because leaves contain not only green chlorophyll, but red and yellow pigments too, just as nature itself reveals them in the Fall when the chlorophyll has faded out. So yes, after determining basic filter factors using a gray card, it is also important to do real-world testing in the field with appropriate subjects.
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