Black Templar vs Omega Blue

How close are they?

ΔE 27.1, loose: different enough that you should pick on purpose. Omega Blue is noticeably lighter, much more saturated than Black Templar.

Can I use Omega Blue instead of Black Templar?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Omega Blue is noticeably lighter, much more saturated than Black Templar.

Can I use Black Templar instead of Omega Blue?

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.