Thanks, Alberto, but I think we may be talking about slightly different things. I was trying not to be too lengthy, and in the process became less clear, maybe. Yes, of course, UV blocking happens in a part of the spectrum we can't see, but this thread has been about how colors (in the visible spectrum) rank as far as blocking UV (in the invisible spectrum) and there's been some difference of observation about the ranking of the colors.
I went through the archives of a couple of forums and generated a whole page-long list of rankings, one theoretical, most empirical, many based in instrument readings, and no two the same. My point was that since different inksets/printers seem to yield different rankings of UV-blocking by color (for some red blocks more than yellow, for some the other way around; for some cyan blocks more than magenta, for some the other way around) then the color can't be the determining factor for ranking the UV-blocking characteristics of the inks, and it would probably be helpful not to make categorical statements about the ranking of colors by UV-blocking.
When I'm referring to which color blocks UV most, I'm not necessarily talking about the color one chooses for a negative; you might or might not want the most UV-blocking color for a negative, depending on other factors. You are apparently using the color patch thingies to determine the negative color; this, like Chris's circles, can give you a very rough ranking of UV blocking of a hue at full strength, but nothing more nuanced than that, and certainly not a quantitative idea of the relative UV-blocking of the different inks. The HSB-HSL arrays give a much better idea, in the amplitude of the peaks and valleys of the profile at hue angles 60 (yellow) 120 (green) 180 (cyan) 240 (blue) and 0/360 (red). It's that profile of the curve formed by the bottom edge of the HSB-HSL array print that I was referring to being the same for the same person using the same printer/inkset/lightsource with different processes, but that person could choose a different negative color from that profile, depending on needs/characteristics of the process.
Instruments are useful for exactly measuring the UV-blocking of a particular ink (I especially like Clay's QTR graphs, which show precisely how much UV a particular ink blocks at a particular percentage) but most of us have to rely on how a thing prints, to judge. Which, come to think of it, is the real goal anyway, isn't it?
When I asked a question on another forum about this a couple of years ago, I was told that the farther the color is from violet on the spectrum, the better it will block UV. By this logic, the UV- blocking capability of colors should be in descending order: red, yellow, green, cyan, blue, magenta. This makes sense theoretically, but in practice it doesn't always seem to come out that way, in fact the ranking of colors by blocking ability seems variable depending more on the printer, the inkset and the light source than on the process.In other -and less- words, it depends on the UV adsorption of the inks and not from the absorption in the visible range. Many animals different from humans behave in this way. We need to think not from what we see (visible spectrum) but from what we don't see (UV range). This is not possible unless, like Marek suggested, we perform a strumental reading.