ΔE 20.9, loose: different enough that you should pick on purpose. Grey Spell is a little lighter, much more muted, warmer than Frostheart.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Grey Spell is a little lighter, much more muted, warmer than Frostheart.
The same difference applies the other way round: Frostheart is a little darker, much more saturated, 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.
See Frostheart on the colour wheel See Grey Spell on the colour wheel
All matches: Frostheart equivalents · Grey Spell equivalents