Dark Sky vs Phthalo Blue (Red Shade)

How close are they?

ΔE 9.3, close: similar, with a visible difference side by side. Phthalo Blue (Red Shade) is a little darker, warmer than Dark Sky. They are different kinds of paint: Army Painter Dark Sky is an opaque base/layer paint and Golden Phthalo Blue (Red Shade) is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.

Can I use Phthalo Blue (Red Shade) instead of Dark Sky?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Phthalo Blue (Red Shade) is a little darker, warmer than Dark Sky. They are different kinds of paint.

Can I use Dark Sky instead of Phthalo Blue (Red Shade)?

The same difference applies the other way round: Dark Sky 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.