After investigating this (and raising the aforementioned Emacs bug), I've concluded that sRGB hex colors - as used by Ethan - are simply not the right hex colors to enter in Emacs in order to get the desired results. Instead, Emacs apparently works with Generic RGB values, and after some laborious conversion calculations in Apple's "ColorSync Utility", I got the right results:

https://github.com/purcell/color-theme-sanityinc-solarized

The color fix will probably get into sellout's repo, so interested parties should prefer that one over mine.