ΔE 19.2, loose: different enough that you should pick on purpose. Flat Brown is noticeably lighter, cooler than Van Dyke Brown Hue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Flat Brown is noticeably lighter, cooler than Van Dyke Brown Hue.
The same difference applies the other way round: Van Dyke Brown Hue is noticeably darker, 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 Van Dyke Brown Hue on the colour wheel See Flat Brown on the colour wheel
All matches: Van Dyke Brown Hue equivalents · Flat Brown equivalents