ΔE 29.4, loose: different enough that you should pick on purpose. Mystic Blue is noticeably darker, warmer than Frostheart.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 29). Mystic Blue is noticeably darker, warmer than Frostheart.
The same difference applies the other way round: Frostheart is noticeably lighter, 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 Mystic Blue on the colour wheel
All matches: Frostheart equivalents · Mystic Blue equivalents