Black Templar vs Wicked Purple

How close are they?

ΔE 24.9, loose: different enough that you should pick on purpose. Wicked Purple is noticeably lighter, more saturated than Black Templar.

Can I use Wicked Purple instead of Black Templar?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Wicked Purple is noticeably lighter, more saturated than Black Templar.

Can I use Black Templar instead of Wicked Purple?

The same difference applies the other way round: Black Templar is noticeably darker, 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.