ΔE 14.7, fair: the same colour family, but not a straight swap. Fluorescent Green is more saturated, cooler than Niblet Green. They are different kinds of paint: Citadel Niblet Green is an opaque base/layer paint and Mr. Hobby Fluorescent Green is a fluorescent or effect 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 15). Fluorescent Green is more saturated, cooler than Niblet Green. They are different kinds of paint.
The same difference applies the other way round: Niblet Green is more muted, 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.
See Niblet Green on the colour wheel See Fluorescent Green on the colour wheel
All matches: Niblet Green equivalents · Fluorescent Green equivalents