Fluorescent Orange-Yellow vs Fluorescent Orange

How close are they?

ΔE 20.7, loose: different enough that you should pick on purpose. Fluorescent Orange is a little darker, cooler than Fluorescent Orange-Yellow.

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

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

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

The same difference applies the other way round: Fluorescent Orange-Yellow 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.