Underhive Ash vs Ironhull Grey

How close are they?

ΔE 19.4, loose: different enough that you should pick on purpose. Ironhull Grey is a little darker, more muted than Underhive Ash.

Can I use Ironhull Grey instead of Underhive Ash?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Ironhull Grey is a little darker, more muted than Underhive Ash.

Can I use Underhive Ash instead of Ironhull Grey?

The same difference applies the other way round: Underhive Ash is a little lighter, 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.