Aethermatic Blue vs Templar White

How close are they?

ΔE 25.4, loose: different enough that you should pick on purpose. Templar White is noticeably lighter, much more muted than Aethermatic Blue.

Can I use Templar White instead of Aethermatic Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Templar White is noticeably lighter, much more muted than Aethermatic Blue.

Can I use Aethermatic Blue instead of Templar White?

The same difference applies the other way round: Aethermatic Blue 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.