Fluorescent Orange-Yellow vs Fluorescent Red

How close are they?

ΔE 41.5, loose: different enough that you should pick on purpose. Fluorescent Red is noticeably darker, more muted, cooler than Fluorescent Orange-Yellow.

Can I use Fluorescent Red instead of Fluorescent Orange-Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 41). Fluorescent Red is noticeably darker, more muted, cooler than Fluorescent Orange-Yellow.

Can I use Fluorescent Orange-Yellow instead of Fluorescent Red?

The same difference applies the other way round: Fluorescent Orange-Yellow is noticeably lighter, more saturated, 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.