Runic Grey vs Frostheart

How close are they?

ΔE 20.1, loose: different enough that you should pick on purpose. Frostheart is a little lighter, much more saturated, warmer than Runic Grey.

Can I use Frostheart instead of Runic Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Frostheart is a little lighter, much more saturated, warmer than Runic Grey.

Can I use Runic Grey instead of Frostheart?

The same difference applies the other way round: Runic Grey is a little darker, much more muted, cooler.

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.