Leigh - unless you are talking hard separation filters, none of these are pure anyway. What I've found very helpful are the old Kodak Wratten color
filter guide, with the published spectrograms of hundreds of different ones, plus fade factors, transmission figures etc. There are cases where the
eye is fooled into seeing a certain color when the filter actually arrives at it in a different manner. Of course, modern glass filters aren't necessarily
the same as Wratten, and might provide their own spectrograms; but it's a valuable resource anyway, and sometimes turns up in used bookstores, or via book search. Then one can compare that to the spectrogram of any unusual film itself, like extended red or attenuated red (ACROS, for example). I find this more important in the lab when doing really fussy film work like color separations and related precision masking than in the field, where the basic published pan filter factors for daylight seem adequate. But whenever I am working with an unfamiliar black and white film I run it through a whole series of filter tests both here at the coast and at high altitude where the shadows are distinctly bluer. I also have a far larger selection of
filters in the lab, including a number of exotic Wratten gels, than I need in the field. I rarely carry more than two contrast filters when backpacking,
not counting a filter or two for color film.