ΔE 7.5, close: similar, with a visible difference side by side. Stonewall Grey is a little darker than MiG-25/MiG-31 Grey.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Stonewall Grey is a little darker than MiG-25/MiG-31 Grey.
The same difference applies the other way round: MiG-25/MiG-31 Grey is a little lighter.
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 MiG-25/MiG-31 Grey on the colour wheel See Stonewall Grey on the colour wheel
All matches: MiG-25/MiG-31 Grey equivalents · Stonewall Grey equivalents