The visible light example you kindly provide us proves that the paper
isn't sensitive to red light at all, nothing else. (Which is pretty much
the case for a single or multigrade b&w paper. The filter below the
enlarger lightsource is red right?)
Yes you are absolutely right. It is a visual that is intended for college age students first encountering colorized negs. It would be an assumption they might have that black ink is what you would use under the enlarger, so the visual dispels that fact as well as indicates roughly what color is used under the enlarger which is very different from what color negatives are used under UV light with Epson printers. So I might show this in the first week of teaching custom negative calibration, but the actual choice of colors they come up with is determined by the PDN color density range palette and their paper and practice and SPT etc. etc. so they may have a more magenta red or whatnot, but it is always in the red "branch" somewhere in the BW darkroom with a multitude of papers and practices. No black ink.
BTW I need to again state/clarify my original point that yellow is the densest STANDALONE ink of the three ink sets but I have never said it is denser than a combination of inks such as found in green (for UV anyway). In other words, I have not used straight yellow negs--ever--for pt/pd as an example. But until you asked your question, Loris, of why the combination of the two (yellow and cyan) which visually seems to not "add up" to hold back as much light, I had always assumed that the color combination was more UV resistant in that process (as related to filtering light which Alberto shot down as having nothing to do with the actual color but everything to do with double bonds etc.) or more ink was laid down by the driver (as you shot down as not being true).
Now, that I have received Clay's chart, I find it fascinating....but I need to clarify this, Clay. Clay, if I am hearing you correctly, your chart shows/proves that UV density is additive when colors are mixed. That much is correct (which dovetails with Alberto's information)? But you also seem to say that there is more ink laid down, is that correct, too? But in your system, you lessen the ink load, correct again? Sorry if these seem so dense but I want to really understand this so I can file it in my brain file cabinet and put it to rest.