ΔE 31.4, loose: different enough that you should pick on purpose. Legacy Blue is noticeably darker, much more muted than Deep Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 31). Legacy Blue is noticeably darker, much more muted than Deep Blue.
The same difference applies the other way round: Deep Blue is noticeably lighter, much more saturated.
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 Deep Blue on the colour wheel See Legacy Blue on the colour wheel
All matches: Deep Blue equivalents · Legacy Blue equivalents