ΔE 6.4, close: similar, with a visible difference side by side. Burnt Sienna Umber is a little darker, warmer than Bootstrap Brown.
Usually, if you are not matching paint already on the model. Close match (ΔE 6). Burnt Sienna Umber is a little darker, warmer than Bootstrap Brown.
The same difference applies the other way round: Bootstrap Brown 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 Bootstrap Brown on the colour wheel See Burnt Sienna Umber on the colour wheel
All matches: Bootstrap Brown equivalents · Burnt Sienna Umber equivalents