ΔE 13.2, fair: the same colour family, but not a straight swap. Mars Yellow is a little lighter, more saturated than Martian Ironearth. They are different kinds of paint: Citadel Martian Ironearth is a technical paint or medium and Golden Mars Yellow 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). Mars Yellow is a little lighter, more saturated than Martian Ironearth. They are different kinds of paint.
The same difference applies the other way round: Martian Ironearth is a little darker, more muted.
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 Ironearth on the colour wheel See Mars Yellow on the colour wheel
All matches: Martian Ironearth equivalents · Mars Yellow equivalents