Fluor Orange vs Fluorescent Red

How close are they?

ΔE 21.2, loose: different enough that you should pick on purpose. Fluorescent Red is a little darker, cooler than Fluor Orange.

Can I use Fluorescent Red instead of Fluor Orange?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Fluorescent Red is a little darker, cooler than Fluor Orange.

Can I use Fluor Orange instead of Fluorescent Red?

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