Fiendish Yellow vs Sandy Yellow

How close are they?

ΔE 21.3, loose: different enough that you should pick on purpose. Sandy Yellow is a little lighter, much more muted, cooler than Fiendish Yellow.

Can I use Sandy Yellow instead of Fiendish Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Sandy Yellow is a little lighter, much more muted, cooler than Fiendish Yellow.

Can I use Fiendish Yellow instead of Sandy Yellow?

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