ΔE 19.2, loose: different enough that you should pick on purpose. Steel Grey is noticeably darker, much more saturated than Sky Grey.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Steel Grey is noticeably darker, much more saturated than Sky Grey.
The same difference applies the other way round: Sky Grey is noticeably lighter, much 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 Sky Grey on the colour wheel See Steel Grey on the colour wheel
All matches: Sky Grey equivalents · Steel Grey equivalents