Runic Grey vs Blue Glacier

How close are they?

ΔE 20.3, loose: different enough that you should pick on purpose. Blue Glacier is noticeably lighter, more saturated than Runic Grey.

Can I use Blue Glacier instead of Runic Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Blue Glacier is noticeably lighter, more saturated than Runic Grey.

Can I use Runic Grey instead of Blue Glacier?

The same difference applies the other way round: Runic 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.