ΔE 35.8, loose: different enough that you should pick on purpose. Mummy White is noticeably lighter, more saturated than Warm Brown.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 36). Mummy White is noticeably lighter, more saturated than Warm Brown.
The same difference applies the other way round: Warm Brown is noticeably 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 Warm Brown on the colour wheel See Mummy White on the colour wheel
All matches: Warm Brown equivalents · Mummy White equivalents