ΔE 7.8, close: similar, with a visible difference side by side. USAF Brown is a little lighter, more saturated than Smoke Gray.
Usually, if you are not matching paint already on the model. Close match (ΔE 8). USAF Brown is a little lighter, more saturated than Smoke Gray.
The same difference applies the other way round: Smoke Gray is a little darker, more muted.
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 Smoke Gray on the colour wheel See USAF Brown on the colour wheel
All matches: Smoke Gray equivalents · USAF Brown equivalents