ΔE 18.2, loose: different enough that you should pick on purpose. Ironhull Grey is a little lighter, much more muted than Nighthaunt Gloom. They are different kinds of paint: Citadel Nighthaunt Gloom is a one-coat contrast-style paint and Formula P3 Ironhull Grey is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Ironhull Grey is a little lighter, much more muted than Nighthaunt Gloom. They are different kinds of paint.
The same difference applies the other way round: Nighthaunt Gloom is a little darker, much more saturated.
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 Nighthaunt Gloom on the colour wheel See Ironhull Grey on the colour wheel
All matches: Nighthaunt Gloom equivalents · Ironhull Grey equivalents