Thanks, PE. It should have occurred to me you'd need different filters for colour negatives and papers. I was thinking too "B&W". I guess ideally I'd use a filter that matches the sensitivity of the paper as closely as possible. Difficult with VC paper as I'd need a blue-green colour. I've designed a few experiments and we'll see what happens.
To use a gel filter with a B&W densitometer, I assume I should zero it with the filter first?
Any idea why it might make a difference of a few density points if the filter is placed between the light source and the emulsion side of the negative (ie below the negative) vs between the base side of the negative and the reading head (ie above the negative)? I'm assuming it must have something to do with small differences in reflection/transmission of light depending on where the gel filter is in the light path, but not sure how. I'm just trying to get it as comparable as possible to the measurements made without the filter.
Of course I'm also assuming a white light reading of a stained negative gives an accurate measurement of silver-only density. A lot of assumptions here...