Fiendish Yellow vs Medium Yellow

How close are they?

ΔE 7.6, close: similar, with a visible difference side by side. Medium Yellow is a little lighter, more saturated than Fiendish Yellow.

Can I use Medium Yellow instead of Fiendish Yellow?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Medium Yellow is a little lighter, more saturated than Fiendish Yellow.

Can I use Fiendish Yellow instead of Medium Yellow?

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

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.