Neon Yellow vs Fluorescent Orange

How close are they?

ΔE 86.8, loose: different enough that you should pick on purpose. Fluorescent Orange is noticeably darker, warmer than Neon Yellow.

Can I use Fluorescent Orange instead of Neon Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 87). Fluorescent Orange is noticeably darker, warmer than Neon Yellow.

Can I use Neon Yellow instead of Fluorescent Orange?

The same difference applies the other way round: Neon Yellow is noticeably lighter, cooler.

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.