Phthalo Green (Blue Shade) vs Park Green Flat

How close are they?

ΔE 18.4, loose: different enough that you should pick on purpose. Park Green Flat is a little darker, much more muted, cooler than Phthalo Green (Blue Shade).

Can I use Park Green Flat instead of Phthalo Green (Blue Shade)?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Park Green Flat is a little darker, much more muted, cooler than Phthalo Green (Blue Shade).

Can I use Phthalo Green (Blue Shade) instead of Park Green Flat?

The same difference applies the other way round: Phthalo Green (Blue Shade) is a little lighter, much more saturated, 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.