ΔE 17.6, loose: different enough that you should pick on purpose. Intense Yellow is noticeably lighter, more saturated, cooler than Fiendish Yellow.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Intense Yellow is noticeably lighter, more saturated, cooler than Fiendish Yellow.
The same difference applies the other way round: Fiendish Yellow is noticeably darker, more muted, 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.
See Fiendish Yellow on the colour wheel See Intense Yellow on the colour wheel
All matches: Fiendish Yellow equivalents · Intense Yellow equivalents