Tallarn Sand vs Mustard Yellow

How close are they?

ΔE 15.4, loose: different enough that you should pick on purpose. Mustard Yellow is a little darker, more saturated, cooler than Tallarn Sand.

Can I use Mustard Yellow instead of Tallarn Sand?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Mustard Yellow is a little darker, more saturated, cooler than Tallarn Sand.

Can I use Tallarn Sand instead of Mustard Yellow?

The same difference applies the other way round: Tallarn Sand is a little lighter, 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.