ΔE 9.4, close: similar, with a visible difference side by side. Flat Brown is a little lighter, more muted, cooler than Umberroot.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Flat Brown is a little lighter, more muted, cooler than Umberroot.
The same difference applies the other way round: Umberroot is a little darker, more saturated, 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 Umberroot on the colour wheel See Flat Brown on the colour wheel
All matches: Umberroot equivalents · Flat Brown equivalents