Trollslayer Orange vs Fluorescent Orange-Yellow

How close are they?

ΔE 15.0, loose: different enough that you should pick on purpose. Fluorescent Orange-Yellow is much more saturated than Trollslayer Orange. They are different kinds of paint: Citadel Trollslayer Orange is an opaque base/layer paint and Golden Fluorescent Orange-Yellow is a fluorescent or effect paint, so they won’t behave the same on the model even where the colour is close.

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

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Fluorescent Orange-Yellow is much more saturated than Trollslayer Orange. They are different kinds of paint.

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

The same difference applies the other way round: Trollslayer Orange is much more muted.

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.