Flickering Flame vs Naples Yellow

How close are they?

ΔE 22.4, loose: different enough that you should pick on purpose. Naples Yellow is noticeably lighter, cooler than Flickering Flame.

Can I use Naples Yellow instead of Flickering Flame?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Naples Yellow is noticeably lighter, cooler than Flickering Flame.

Can I use Flickering Flame instead of Naples Yellow?

The same difference applies the other way round: Flickering Flame is noticeably darker, 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.