Yeh photoshop for some reason handles greyscale different than promotion. dunno why that is so, the colours should be same luma as said in pepto's article and promotion does that with the original pepto palette and i adjusted this one to have those lumapairs as well.
Converting a color to grey doesn't guarantee luma-correctness; the only thing that can guarantee that is having an appropriate color profile for the source image and an appropriate profile for your monitor.
Otherwise, it's just a semi-dumb weighting - whether linear, as in the following formula:
#define GIMP_RGB_LUMINANCE_RED (0.2126)
#define GIMP_RGB_LUMINANCE_GREEN (0.7152)
#define GIMP_RGB_LUMINANCE_BLUE (0.0722)
#define GIMP_RGB_LUMINANCE(r,g,b) ((r) * GIMP_RGB_LUMINANCE_RED + \
(g) * GIMP_RGB_LUMINANCE_GREEN + \
(b) * GIMP_RGB_LUMINANCE_BLUE)
or nonlinear, as in L*a*b colorspace.
Gimp recognizes this, and provides several options for how to convert to greyscale. I would be surprised if photoshop didn't too.
Here are some different algorithyms lined up. For me, gimp's estimate of luminance matches the original colors more closely than the greys shown before.
Particularly, I think it estimates the greens,cyan, and dark pink markedly better.
'L of L*a*b' achieves the best idea of the colors relation to each other (one of L*a*b's strengths) and is probably the most useful for reference purposes.
Fool: I'd suggest just non-dithered black. alternatively, a non-standard dither pattern blue/brown lightening some areas to emphasize the darkness of the blue.