ΔE 21.8, loose: different enough that you should pick on purpose. Frostheart is noticeably lighter, more muted, cooler than Highlord Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Frostheart is noticeably lighter, more muted, cooler than Highlord Blue.
The same difference applies the other way round: Highlord Blue is noticeably darker, more saturated, warmer.
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 Highlord Blue on the colour wheel See Frostheart on the colour wheel
All matches: Highlord Blue equivalents · Frostheart equivalents