ΔE 10.3, fair: the same colour family, but not a straight swap. Sapphire Blue is a little darker, warmer than Battleship Grey. They are different kinds of paint: Army Painter Battleship Grey is a one-coat contrast-style paint and Green Stuff World Sapphire Blue 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). Sapphire Blue is a little darker, warmer than Battleship Grey. They are different kinds of paint.
The same difference applies the other way round: Battleship Grey is a little lighter, 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 Battleship Grey on the colour wheel See Sapphire Blue on the colour wheel
All matches: Battleship Grey equivalents · Sapphire Blue equivalents