Nightshade Purple vs Mystic Blue

How close are they?

ΔE 19.3, loose: different enough that you should pick on purpose. Mystic Blue is a little lighter, more saturated, cooler than Nightshade Purple.

Can I use Mystic Blue instead of Nightshade Purple?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Mystic Blue is a little lighter, more saturated, cooler than Nightshade Purple.

Can I use Nightshade Purple instead of Mystic Blue?

The same difference applies the other way round: Nightshade Purple is a little darker, more muted, 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.