Rotten Pus vs Imperial Yellow

How close are they?

ΔE 22.7, loose: different enough that you should pick on purpose. Imperial Yellow is a little darker, more muted, warmer than Rotten Pus.

Can I use Imperial Yellow instead of Rotten Pus?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Imperial Yellow is a little darker, more muted, warmer than Rotten Pus.

Can I use Rotten Pus instead of Imperial Yellow?

The same difference applies the other way round: Rotten Pus is a little lighter, more saturated, 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.