ΔE 16.7, loose: different enough that you should pick on purpose. Light Phthalo Blue is noticeably lighter, more saturated than Frost Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Light Phthalo Blue is noticeably lighter, more saturated than Frost Blue.
The same difference applies the other way round: Frost Blue is noticeably darker, more muted.
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 Frost Blue on the colour wheel See Light Phthalo Blue on the colour wheel
All matches: Frost Blue equivalents · Light Phthalo Blue equivalents