Cobalt Blue vs Imperial Purple

How close are they?

ΔE 23.1, loose: different enough that you should pick on purpose. Imperial Purple is noticeably lighter, warmer than Cobalt Blue.

Can I use Imperial Purple instead of Cobalt Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Imperial Purple is noticeably lighter, warmer than Cobalt Blue.

Can I use Cobalt Blue instead of Imperial Purple?

The same difference applies the other way round: Cobalt Blue is noticeably darker, cooler.

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.