ΔE 14.7, fair: the same colour family, but not a straight swap. Warpfiend Grey is much more muted, warmer than Matt Ultra Blue. They are different kinds of paint: AMMO by Mig Matt Ultra Blue is an ink, glaze or transparent paint and Citadel Warpfiend Grey is an opaque base/layer 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 15). Warpfiend Grey is much more muted, warmer than Matt Ultra Blue. They are different kinds of paint.
The same difference applies the other way round: Matt Ultra Blue is much 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 Matt Ultra Blue on the colour wheel See Warpfiend Grey on the colour wheel
All matches: Matt Ultra Blue equivalents · Warpfiend Grey equivalents