Black Templar vs Green Stone

How close are they?

ΔE 19.1, loose: different enough that you should pick on purpose. Green Stone is noticeably lighter, more saturated than Black Templar.

Can I use Green Stone instead of Black Templar?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Green Stone is noticeably lighter, more saturated than Black Templar.

Can I use Black Templar instead of Green Stone?

The same difference applies the other way round: Black Templar is noticeably darker, 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.