ΔE 12.3, fair: the same colour family, but not a straight swap. Underhive Ash is a little lighter, cooler than Zimmerit.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Underhive Ash is a little lighter, cooler than Zimmerit.
The same difference applies the other way round: Zimmerit is a little darker, warmer.
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 Zimmerit on the colour wheel See Underhive Ash on the colour wheel
All matches: Zimmerit equivalents · Underhive Ash equivalents