ΔE 25.4, loose: different enough that you should pick on purpose. Undead Dragon is noticeably lighter, much more muted, cooler than Rigor Mortis.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Undead Dragon is noticeably lighter, much more muted, cooler than Rigor Mortis.
The same difference applies the other way round: Rigor Mortis is noticeably darker, much more saturated, warmer.
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 Undead Dragon on the colour wheel
All matches: Rigor Mortis equivalents · Undead Dragon equivalents