ΔE 18.7, loose: different enough that you should pick on purpose. Estus Yellow is a little lighter, more muted, warmer than Malignant Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Estus Yellow is a little lighter, more muted, warmer than Malignant Green.
The same difference applies the other way round: Malignant Green is a little darker, 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.
See Malignant Green on the colour wheel See Estus Yellow on the colour wheel
All matches: Malignant Green equivalents · Estus Yellow equivalents