Forest of Shadows vs Foul Green

How close are they?

ΔE 28.1, loose: different enough that you should pick on purpose. Foul Green is noticeably lighter, more saturated than Forest of Shadows.

Can I use Foul Green instead of Forest of Shadows?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Foul Green is noticeably lighter, more saturated than Forest of Shadows.

Can I use Forest of Shadows instead of Foul Green?

The same difference applies the other way round: Forest of Shadows 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.