ΔE 26.4, loose: different enough that you should pick on purpose. Mystic Blue is much more muted, cooler than Deep Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Mystic Blue is much more muted, cooler than Deep Blue.
The same difference applies the other way round: Deep Blue is much 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 Deep Blue on the colour wheel See Mystic Blue on the colour wheel
All matches: Deep Blue equivalents · Mystic Blue equivalents