Darling Mark (that must be what the m is for
Thank you, I understand. I got thrown off by this experiment: Create a new document in ProPhoto RGB 16-bit, fill it with fully saturated yellow, RGB 255, 255, 0, Lab 100, -5, 128. Now assign printer profile Epson Velvet Fine Art, the result is RGB 255, 255, 0, Lab 95, -1, 117. This point is actually outside the printer gamut, as seen in the plot.
If instead, I assign profile sRGB the resulting Lab is 98, -16, 93, which plots right at the vertex of sRGB space, as you would expect.
So I wondered if somehow printer profiles were different. But you are right, RGB spaces by definition are defined by their three primaries and white point.
I have since tried other saturated colors, such as cyan, and they plot within gamut, both for the Velvet Fine Art profile, and the Hahnemuhle Fine Art Baryta profile. So the yellow point I started with is probably ok; it shows outside the PerfX gamut display, but that may be due to their calculations.
So I barked up the wrong tree.
I was curious about your statement that the Lab values are the same before and after conversion. If this is true for the patches that plot outside the printer gamut in ColorThink, then the problem most likely is with ColorThink, no? If they are different, most likely PS. Since ColorThink is looking into it, you should get the answer. Hope you can let us know.