WWI French Brown vs Doombull Brown

How close are they?

ΔE 15.7, loose: different enough that you should pick on purpose. Doombull Brown is more saturated, cooler than WWI French Brown.

Can I use Doombull Brown instead of WWI French Brown?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Doombull Brown is more saturated, cooler than WWI French Brown.

Can I use WWI French Brown instead of Doombull Brown?

The same difference applies the other way round: WWI French Brown is more muted, 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.