Fluorescent Orange vs Red Fluor

How close are they?

ΔE 16.8, loose: different enough that you should pick on purpose. Red Fluor is a little darker, more saturated, cooler than Fluorescent Orange.

Can I use Red Fluor instead of Fluorescent Orange?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Red Fluor is a little darker, more saturated, cooler than Fluorescent Orange.

Can I use Fluorescent Orange instead of Red Fluor?

The same difference applies the other way round: Fluorescent Orange is a little lighter, 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.