ΔE 22.5, loose: different enough that you should pick on purpose. Skull Linen is noticeably lighter, much more muted than Runic Grey.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Skull Linen is noticeably lighter, much more muted than Runic Grey.
The same difference applies the other way round: Runic Grey is noticeably darker, much more saturated.
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 Runic Grey on the colour wheel See Skull Linen on the colour wheel
All matches: Runic Grey equivalents · Skull Linen equivalents