Russian Green vs Abaddon Black

How close are they?

ΔE 15.8, loose: different enough that you should pick on purpose. Abaddon Black is a little darker, much more muted than Russian Green.

Can I use Abaddon Black instead of Russian Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Abaddon Black is a little darker, much more muted than Russian Green.

Can I use Russian Green instead of Abaddon Black?

The same difference applies the other way round: Russian Green 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.