Periwinkle Purple vs Deep Blue

How close are they?

ΔE 17.9, loose: different enough that you should pick on purpose. Deep Blue is noticeably darker, much more saturated than Periwinkle Purple.

Can I use Deep Blue instead of Periwinkle Purple?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Deep Blue is noticeably darker, much more saturated than Periwinkle Purple.

Can I use Periwinkle Purple instead of Deep Blue?

The same difference applies the other way round: Periwinkle Purple is noticeably lighter, much more muted.

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.