Alien Purple vs Phthalo Blue (Green Shade)

How close are they?

ΔE 20.7, loose: different enough that you should pick on purpose. Phthalo Blue (Green Shade) is much more saturated, cooler than Alien Purple.

Can I use Phthalo Blue (Green Shade) instead of Alien Purple?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Phthalo Blue (Green Shade) is much more saturated, cooler than Alien Purple.

Can I use Alien Purple instead of Phthalo Blue (Green Shade)?

The same difference applies the other way round: Alien Purple is much more muted, warmer.

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.