Autumn Sage vs Nuclear Sludge

How close are they?

ΔE 15.8, loose: different enough that you should pick on purpose. Nuclear Sludge is a little darker, much more saturated, warmer than Autumn Sage.

Can I use Nuclear Sludge instead of Autumn Sage?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Nuclear Sludge is a little darker, much more saturated, warmer than Autumn Sage.

Can I use Autumn Sage instead of Nuclear Sludge?

The same difference applies the other way round: Autumn Sage is a little lighter, much more muted, cooler.

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.