ΔE 16.9, loose: different enough that you should pick on purpose. Skeleton Horde is noticeably lighter, warmer than Rigor Mortis.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Skeleton Horde is noticeably lighter, warmer than Rigor Mortis.
The same difference applies the other way round: Rigor Mortis is noticeably darker, cooler.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Rigor Mortis on the colour wheel See Skeleton Horde on the colour wheel
All matches: Rigor Mortis equivalents · Skeleton Horde equivalents