ΔE 6.3, close: similar, with a visible difference side by side. Acid is a little lighter than Niblet Green. They are different kinds of paint: Citadel Niblet Green is an opaque base/layer paint and Vallejo Acid is a fluorescent or effect paint, so they won’t behave the same on the model even where the colour is close.
Usually, if you are not matching paint already on the model. Close match (ΔE 6). Acid is a little lighter than Niblet Green. They are different kinds of paint.
The same difference applies the other way round: Niblet Green is a little darker.
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 Acid on the colour wheel
All matches: Niblet Green equivalents · Acid equivalents