ΔE 10.2, fair: the same colour family, but not a straight swap. Payne's Gray is a little darker, more muted, warmer than The Fang. They are different kinds of paint: Citadel The Fang is an opaque base/layer paint and Golden Payne's Gray 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 10). Payne's Gray is a little darker, more muted, warmer than The Fang. They are different kinds of paint.
The same difference applies the other way round: The Fang is a little lighter, more saturated, cooler.
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 The Fang on the colour wheel See Payne's Gray on the colour wheel
All matches: The Fang equivalents · Payne's Gray equivalents