ΔE 10.1, fair: the same colour family, but not a straight swap. Iron Fist is a little darker than Grey Knights Steel.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 10). Iron Fist is a little darker than Grey Knights Steel.
The same difference applies the other way round: Grey Knights Steel is a little 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 Grey Knights Steel on the colour wheel See Iron Fist on the colour wheel
All matches: Grey Knights Steel equivalents · Iron Fist equivalents