Freezing Moon vs Grey Seer

How close are they?

ΔE 18.7, loose: different enough that you should pick on purpose. Grey Seer is noticeably lighter, more muted than Freezing Moon.

Can I use Grey Seer instead of Freezing Moon?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Grey Seer is noticeably lighter, more muted than Freezing Moon.

Can I use Freezing Moon instead of Grey Seer?

The same difference applies the other way round: Freezing Moon is noticeably darker, 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.