Black Templar vs Deep Purple

How close are they?

ΔE 31.5, loose: different enough that you should pick on purpose. Deep Purple is noticeably lighter, much more saturated than Black Templar.

Can I use Deep Purple instead of Black Templar?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 32). Deep Purple is noticeably lighter, much more saturated than Black Templar.

Can I use Black Templar instead of Deep Purple?

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