ΔE 9.6, close: similar, with a visible difference side by side. Plague Brown is a little lighter, more muted, cooler than Fiendish Yellow.
Usually, if you are not matching paint already on the model. Close match (ΔE 10). Plague Brown is a little lighter, more muted, cooler than Fiendish Yellow.
The same difference applies the other way round: Fiendish Yellow is a little darker, more saturated, warmer.
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 Fiendish Yellow on the colour wheel See Plague Brown on the colour wheel
All matches: Fiendish Yellow equivalents · Plague Brown equivalents