Dark Turquoise vs Russian AF Grey N.7

How close are they?

ΔE 17.6, loose: different enough that you should pick on purpose. Russian AF Grey N.7 is noticeably lighter, more muted than Dark Turquoise.

Can I use Russian AF Grey N.7 instead of Dark Turquoise?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Russian AF Grey N.7 is noticeably lighter, more muted than Dark Turquoise.

Can I use Dark Turquoise instead of Russian AF Grey N.7?

The same difference applies the other way round: Dark Turquoise is noticeably 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.