ΔE 10.7, fair: the same colour family, but not a straight swap. Tenebris Blue is a little lighter, warmer than Caledor Sky. They are different kinds of paint: Citadel Caledor Sky is an opaque base/layer paint and Green Stuff World Tenebris 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 11). Tenebris Blue is a little lighter, warmer than Caledor Sky. They are different kinds of paint.
The same difference applies the other way round: Caledor Sky is a little darker, 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 Caledor Sky on the colour wheel See Tenebris Blue on the colour wheel
All matches: Caledor Sky equivalents · Tenebris Blue equivalents