Green Tone vs Oblivion Black Wash

How close are they?

ΔE 42.7, loose: different enough that you should pick on purpose. Oblivion Black Wash is noticeably lighter, much more muted than Green Tone.

Can I use Oblivion Black Wash instead of Green Tone?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 43). Oblivion Black Wash is noticeably lighter, much more muted than Green Tone.

Can I use Green Tone instead of Oblivion Black Wash?

The same difference applies the other way round: Green Tone is noticeably darker, 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.