ΔE 6.2, close: similar, with a visible difference side by side. Mortarion Grime is a little darker, cooler than IDF Sand Grey 1970s-1980s. They are different kinds of paint: AK Interactive IDF Sand Grey 1970s-1980s is an opaque base/layer paint and Citadel Mortarion Grime is a wash/shade, 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 6). Mortarion Grime is a little darker, cooler than IDF Sand Grey 1970s-1980s. They are different kinds of paint.
The same difference applies the other way round: IDF Sand Grey 1970s-1980s is a little lighter, 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 IDF Sand Grey 1970s-1980s on the colour wheel See Mortarion Grime on the colour wheel
All matches: IDF Sand Grey 1970s-1980s equivalents · Mortarion Grime equivalents