Pastel Seafoam vs Templar White

How close are they?

ΔE 15.1, loose: different enough that you should pick on purpose. Templar White is a little darker, much more muted than Pastel Seafoam.

Can I use Templar White instead of Pastel Seafoam?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Templar White is a little darker, much more muted than Pastel Seafoam.

Can I use Pastel Seafoam instead of Templar White?

The same difference applies the other way round: Pastel Seafoam is a little lighter, 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.