Fiendish Yellow vs Ember Orange

How close are they?

ΔE 18.9, loose: different enough that you should pick on purpose. Ember Orange is a little darker, more muted, warmer than Fiendish Yellow.

Can I use Ember Orange instead of Fiendish Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Ember Orange is a little darker, more muted, warmer than Fiendish Yellow.

Can I use Fiendish Yellow instead of Ember Orange?

The same difference applies the other way round: Fiendish Yellow is a little lighter, more saturated, 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.