Orchid vs Witch Purple

How close are they?

ΔE 23.1, loose: different enough that you should pick on purpose. Witch Purple is more saturated, warmer than Orchid.

Can I use Witch Purple instead of Orchid?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Witch Purple is more saturated, warmer than Orchid.

Can I use Orchid instead of Witch Purple?

The same difference applies the other way round: Orchid is more muted, 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.