Rigor Mortis vs Orc Skin

How close are they?

ΔE 27.8, loose: different enough that you should pick on purpose. Orc Skin is noticeably darker, more saturated, cooler than Rigor Mortis.

Can I use Orc Skin instead of Rigor Mortis?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Orc Skin is noticeably darker, more saturated, cooler than Rigor Mortis.

Can I use Rigor Mortis instead of Orc Skin?

The same difference applies the other way round: Rigor Mortis is noticeably lighter, more muted, 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.