ΔE 12.4, fair: the same colour family, but not a straight swap. Decayed Metal is noticeably darker than Iron Warriors.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Decayed Metal is noticeably darker than Iron Warriors.
The same difference applies the other way round: Iron Warriors is noticeably lighter.
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 Iron Warriors on the colour wheel See Decayed Metal on the colour wheel
All matches: Iron Warriors equivalents · Decayed Metal equivalents