Dark Feldgrau vs Tenebrous Teal

How close are they?

ΔE 17.1, loose: different enough that you should pick on purpose. Tenebrous Teal is a little darker, more muted than Dark Feldgrau.

Can I use Tenebrous Teal instead of Dark Feldgrau?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Tenebrous Teal is a little darker, more muted than Dark Feldgrau.

Can I use Dark Feldgrau instead of Tenebrous Teal?

The same difference applies the other way round: Dark Feldgrau is a little lighter, 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.