Dark Purple vs Storm Blue

How close are they?

ΔE 17.8, loose: different enough that you should pick on purpose. Storm Blue is noticeably lighter, cooler than Dark Purple.

Can I use Storm Blue instead of Dark Purple?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Storm Blue is noticeably lighter, cooler than Dark Purple.

Can I use Dark Purple instead of Storm Blue?

The same difference applies the other way round: Dark Purple is noticeably darker, 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.