Fiendish Yellow vs Heartfire

How close are they?

ΔE 6.4, close: similar, with a visible difference side by side. Heartfire is a little lighter, more muted than Fiendish Yellow.

Can I use Heartfire instead of Fiendish Yellow?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Heartfire is a little lighter, more muted than Fiendish Yellow.

Can I use Fiendish Yellow instead of Heartfire?

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

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.