ΔE 23.8, loose: different enough that you should pick on purpose. Ink Sepia is noticeably darker, more muted, cooler than Walnut Brown.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Ink Sepia is noticeably darker, more muted, cooler than Walnut Brown.
The same difference applies the other way round: Walnut Brown is noticeably lighter, more saturated, 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 Walnut Brown on the colour wheel See Ink Sepia on the colour wheel
All matches: Walnut Brown equivalents · Ink Sepia equivalents