ΔE 16.8, loose: different enough that you should pick on purpose. Sombre Grey is noticeably lighter, more saturated than Slate Grey.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Sombre Grey is noticeably lighter, more saturated than Slate Grey.
The same difference applies the other way round: Slate Grey is noticeably darker, more muted.
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 Slate Grey on the colour wheel See Sombre Grey on the colour wheel
All matches: Slate Grey equivalents · Sombre Grey equivalents