ΔE 13.3, fair: the same colour family, but not a straight swap. Bestial Brown is a little darker, more saturated, warmer than Martian Ironcrust. They are different kinds of paint: Citadel Martian Ironcrust is a technical paint or medium and Green Stuff World Bestial Brown is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Bestial Brown is a little darker, more saturated, warmer than Martian Ironcrust. They are different kinds of paint.
The same difference applies the other way round: Martian Ironcrust 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.
See Martian Ironcrust on the colour wheel See Bestial Brown on the colour wheel
All matches: Martian Ironcrust equivalents · Bestial Brown equivalents