Orchid vs Imperial Purple

How close are they?

ΔE 19.2, loose: different enough that you should pick on purpose. Imperial Purple is a little darker, cooler than Orchid.

Can I use Imperial Purple instead of Orchid?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Imperial Purple is a little darker, cooler than Orchid.

Can I use Orchid instead of Imperial Purple?

The same difference applies the other way round: Orchid is a little lighter, 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.