ΔE 9.0, close: similar, with a visible difference side by side. Mahogany Brown is a little darker, warmer than Stonewall Grey.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Mahogany Brown is a little darker, warmer than Stonewall Grey.
The same difference applies the other way round: Stonewall Grey 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 Stonewall Grey on the colour wheel See Mahogany Brown on the colour wheel
All matches: Stonewall Grey equivalents · Mahogany Brown equivalents