ΔE 7.0, close: similar, with a visible difference side by side. IDF Sand Grey is a little darker, warmer than Underhive Ash.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). IDF Sand Grey is a little darker, warmer than Underhive Ash.
The same difference applies the other way round: Underhive Ash is a little lighter, cooler.
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 Underhive Ash on the colour wheel See IDF Sand Grey on the colour wheel
All matches: Underhive Ash equivalents · IDF Sand Grey equivalents