Nuclear Sludge vs Foul Green

How close are they?

ΔE 18.4, loose: different enough that you should pick on purpose. Foul Green is a little lighter, more saturated, cooler than Nuclear Sludge.

Can I use Foul Green instead of Nuclear Sludge?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Foul Green is a little lighter, more saturated, cooler than Nuclear Sludge.

Can I use Nuclear Sludge instead of Foul Green?

The same difference applies the other way round: Nuclear Sludge is a little darker, more muted, warmer.

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.