ΔE 21.3, loose: different enough that you should pick on purpose. Sand Grey is noticeably lighter, more saturated than Grey Seer.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Sand Grey is noticeably lighter, more saturated than Grey Seer.
The same difference applies the other way round: Grey Seer 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 Grey Seer on the colour wheel See Sand Grey on the colour wheel
All matches: Grey Seer equivalents · Sand Grey equivalents