Mustard Yellow vs Leather Brown

How close are they?

ΔE 16.7, loose: different enough that you should pick on purpose. Leather Brown is a little lighter, more saturated, warmer than Mustard Yellow.

Can I use Leather Brown instead of Mustard Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Leather Brown is a little lighter, more saturated, warmer than Mustard Yellow.

Can I use Mustard Yellow instead of Leather Brown?

The same difference applies the other way round: Mustard Yellow is a little darker, 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.