The real problem is the web color uses the same name, but not the same color. X11 'gray' is rather light (#BEBEBE / 75%), where the W3C 'gray' is significantly

The real problem is the web color uses the same name, but not the same color. X11 'gray' is rather light (#BEBEBE / 75%), where the W3C 'gray' is significantly darker (#808080 / 50%). (X11 also had a 'web gray' that matched this!) The X11 'dark gray' at 66% (rather like the W3C 'silver') thus made sense in its color set, though not when imported into a different set. You might say, "this means X11 only had very light grays", and from a named perspective this seems to be true. I'm not certain if this is due to UI fashion or a physical artefact of the monitors. But in any case, if you wanted darker shades in X11, you could use 'grayNN', from grey0 (black) on up to grey100 (white).