Yellow Dune vs Mustard Yellow

How close are they?

ΔE 15.7, loose: different enough that you should pick on purpose. Mustard Yellow is much more saturated, cooler than Yellow Dune.

Can I use Mustard Yellow instead of Yellow Dune?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Mustard Yellow is much more saturated, cooler than Yellow Dune.

Can I use Yellow Dune instead of Mustard Yellow?

The same difference applies the other way round: Yellow Dune is much more muted, 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.