ΔE 12.7, fair: the same colour family, but not a straight swap. Nihilakh Oxide is a little darker, warmer than Sky Blue. They are different kinds of paint: AMMO by Mig Sky Blue is an ink, glaze or transparent paint and Citadel Nihilakh Oxide is a technical paint or medium, 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 13). Nihilakh Oxide is a little darker, warmer than Sky Blue. They are different kinds of paint.
The same difference applies the other way round: Sky Blue 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 Sky Blue on the colour wheel See Nihilakh Oxide on the colour wheel
All matches: Sky Blue equivalents · Nihilakh Oxide equivalents