Frostheart vs Iceberg Grey

How close are they?

ΔE 33.7, loose: different enough that you should pick on purpose. Iceberg Grey is noticeably darker, much more muted, warmer than Frostheart.

Can I use Iceberg Grey instead of Frostheart?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 34). Iceberg Grey is noticeably darker, much more muted, warmer than Frostheart.

Can I use Frostheart instead of Iceberg Grey?

The same difference applies the other way round: Frostheart is noticeably lighter, 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.