ΔE 10.8, fair: the same colour family, but not a straight swap. Fluor Yellow is more muted than Flash Gitz Yellow. They are different kinds of paint: Citadel Flash Gitz Yellow is an opaque base/layer paint and Green Stuff World Fluor Yellow is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Fluor Yellow is more muted than Flash Gitz Yellow. They are different kinds of paint.
The same difference applies the other way round: Flash Gitz Yellow is more saturated.
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.
See Flash Gitz Yellow on the colour wheel See Fluor Yellow on the colour wheel
All matches: Flash Gitz Yellow equivalents · Fluor Yellow equivalents