Dark Sky vs Phthalo Blue RS

How close are they?

ΔE 15.1, loose: different enough that you should pick on purpose. Phthalo Blue RS is much more saturated, warmer than Dark Sky.

Can I use Phthalo Blue RS instead of Dark Sky?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Phthalo Blue RS is much more saturated, warmer than Dark Sky.

Can I use Dark Sky instead of Phthalo Blue RS?

The same difference applies the other way round: Dark Sky is much more muted, 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.