Malignant Green vs Imperial Yellow

How close are they?

ΔE 54.7, loose: different enough that you should pick on purpose. Imperial Yellow is noticeably darker, warmer than Malignant Green.

Can I use Imperial Yellow instead of Malignant Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 55). Imperial Yellow is noticeably darker, warmer than Malignant Green.

Can I use Malignant Green instead of Imperial Yellow?

The same difference applies the other way round: Malignant Green is noticeably lighter, 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.