ΔE 16.6, loose: different enough that you should pick on purpose. Payne's Gray is a little lighter, much more muted than Blue Tone. They are different kinds of paint: Army Painter Blue Tone is a wash/shade 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.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Payne's Gray is a little lighter, much more muted than Blue Tone. They are different kinds of paint.
The same difference applies the other way round: Blue Tone is a little darker, much 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 Blue Tone on the colour wheel See Payne's Gray on the colour wheel
All matches: Blue Tone equivalents · Payne's Gray equivalents