Malignant Green vs Warm Yellow

How close are they?

ΔE 31.6, loose: different enough that you should pick on purpose. Warm Yellow is a little darker, more saturated, warmer than Malignant Green.

Can I use Warm Yellow instead of Malignant Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 32). Warm Yellow is a little darker, more saturated, warmer than Malignant Green.

Can I use Malignant Green instead of Warm Yellow?

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