Nuclear Sludge vs Sybarite Green

How close are they?

ΔE 15.4, loose: different enough that you should pick on purpose. Sybarite Green is noticeably darker, cooler than Nuclear Sludge.

Can I use Sybarite Green instead of Nuclear Sludge?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Sybarite Green is noticeably darker, cooler than Nuclear Sludge.

Can I use Nuclear Sludge instead of Sybarite Green?

The same difference applies the other way round: Nuclear Sludge is noticeably lighter, 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.