Dark Purple vs Mystic Blue

How close are they?

ΔE 24.6, loose: different enough that you should pick on purpose. Mystic Blue is noticeably lighter, more muted, cooler than Dark Purple.

Can I use Mystic Blue instead of Dark Purple?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Mystic Blue is noticeably lighter, more muted, cooler than Dark Purple.

Can I use Dark Purple instead of Mystic Blue?

The same difference applies the other way round: Dark Purple 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.