ΔE 20.0, loose: different enough that you should pick on purpose. Dark Umber is a little darker, much more muted, warmer than Doombull Brown.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Dark Umber is a little darker, much more muted, warmer than Doombull Brown.
The same difference applies the other way round: Doombull Brown is a little lighter, much more saturated, 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 Doombull Brown on the colour wheel See Dark Umber on the colour wheel
All matches: Doombull Brown equivalents · Dark Umber equivalents