Red Fluor vs Neon Yellow

How close are they?

ΔE 100.5, loose: different enough that you should pick on purpose. Neon Yellow is noticeably lighter, more saturated, cooler than Red Fluor.

Can I use Neon Yellow instead of Red Fluor?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 100). Neon Yellow is noticeably lighter, more saturated, cooler than Red Fluor.

Can I use Red Fluor instead of Neon Yellow?

The same difference applies the other way round: Red Fluor 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.