ΔE 11.3, fair: the same colour family, but not a straight swap. Payne's Gray is more saturated than Mechanicus Standard Grey. They are different kinds of paint: Citadel Mechanicus Standard Grey 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 11). Payne's Gray is more saturated than Mechanicus Standard Grey. They are different kinds of paint.
The same difference applies the other way round: Mechanicus Standard Grey is more muted.
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 Mechanicus Standard Grey on the colour wheel See Payne's Gray on the colour wheel
All matches: Mechanicus Standard Grey equivalents · Payne's Gray equivalents