ΔE 10.9, fair: the same colour family, but not a straight swap. Nighthaunt Gloom is a little lighter, more muted, warmer than Duck Blue. They are different kinds of paint: AMMO by Mig Duck Blue is an opaque base/layer paint and Citadel Nighthaunt Gloom 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). Nighthaunt Gloom is a little lighter, more muted, warmer than Duck Blue. They are different kinds of paint.
The same difference applies the other way round: Duck Blue is a little darker, more saturated, 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 Duck Blue on the colour wheel See Nighthaunt Gloom on the colour wheel
All matches: Duck Blue equivalents · Nighthaunt Gloom equivalents