ΔE 10.0, close: similar, with a visible difference side by side. Underhive Ash is cooler than Grime. They are different kinds of paint: AMMO by Mig Grime is an ink, glaze or transparent paint and Citadel Underhive Ash is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
Usually, if you are not matching paint already on the model. Close match (ΔE 10). Underhive Ash is cooler than Grime. They are different kinds of paint.
The same difference applies the other way round: Grime is 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 Grime on the colour wheel See Underhive Ash on the colour wheel
All matches: Grime equivalents · Underhive Ash equivalents