ΔE 4.2, very close: most people won’t see a difference side by side. Our swatches for the two are almost the same. They are different kinds of paint: Citadel Martian Ironcrust is a technical paint or medium and Green Stuff World Siena is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Yes, for most jobs. Very close match (ΔE 4). The swatches are almost the same. They are different kinds of paint.
The same difference applies the other way round.
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 Siena on the colour wheel
All matches: Martian Ironcrust equivalents · Siena equivalents