ΔE 18.1, loose: different enough that you should pick on purpose. Leather Brown is a little darker, more saturated, warmer than Viper Brown.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Leather Brown is a little darker, more saturated, warmer than Viper Brown.
The same difference applies the other way round: Viper Brown is a little lighter, more muted, 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 Viper Brown on the colour wheel See Leather Brown on the colour wheel
All matches: Viper Brown equivalents · Leather Brown equivalents