ΔE 10.6, fair: the same colour family, but not a straight swap. Battleship Grey is noticeably darker, cooler than Matt Sky Blue. They are different kinds of paint: AMMO by Mig Matt Sky Blue is an ink, glaze or transparent paint and Army Painter Battleship Grey is a one-coat contrast-style 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). Battleship Grey is noticeably darker, cooler than Matt Sky Blue. They are different kinds of paint.
The same difference applies the other way round: Matt Sky Blue is noticeably lighter, warmer.
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 Matt Sky Blue on the colour wheel See Battleship Grey on the colour wheel
All matches: Matt Sky Blue equivalents · Battleship Grey equivalents