Fiendish Yellow vs Mustard Yellow

How close are they?

ΔE 16.2, loose: different enough that you should pick on purpose. Mustard Yellow is more muted, cooler than Fiendish Yellow.

Can I use Mustard Yellow instead of Fiendish Yellow?

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

Can I use Fiendish Yellow instead of Mustard Yellow?

The same difference applies the other way round: Fiendish Yellow is 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.