Dark Turquoise vs Templar White

How close are they?

ΔE 43.7, loose: different enough that you should pick on purpose. Templar White is noticeably lighter, much more muted than Dark Turquoise.

Can I use Templar White instead of Dark Turquoise?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 44). Templar White is noticeably lighter, much more muted than Dark Turquoise.

Can I use Dark Turquoise instead of Templar White?

The same difference applies the other way round: Dark Turquoise is noticeably darker, much more saturated.

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.