Ashen Stone vs Templar White

How close are they?

ΔE 8.5, close: similar, with a visible difference side by side. Templar White is a little lighter, more muted than Ashen Stone.

Can I use Templar White instead of Ashen Stone?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Templar White is a little lighter, more muted than Ashen Stone.

Can I use Ashen Stone instead of Templar White?

The same difference applies the other way round: Ashen Stone is a little darker, 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.