ΔE 14.4, fair: the same colour family, but not a straight swap. Iron Hue is noticeably lighter than Necron Compound.
Can I use Iron Hue instead of Necron Compound?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Iron Hue is noticeably lighter than Necron Compound.
Can I use Necron Compound instead of Iron Hue?
The same difference applies the other way round: Necron Compound is noticeably darker.
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.