Fiendish Yellow vs Meaty Ochre

How close are they?

ΔE 23.8, loose: different enough that you should pick on purpose. Meaty Ochre is noticeably darker, much more muted than Fiendish Yellow.

Can I use Meaty Ochre instead of Fiendish Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Meaty Ochre is noticeably darker, much more muted than Fiendish Yellow.

Can I use Fiendish Yellow instead of Meaty Ochre?

The same difference applies the other way round: Fiendish Yellow is noticeably lighter, much 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.