Royal Cloak vs Phthalo Green

How close are they?

ΔE 19.0, loose: different enough that you should pick on purpose. Phthalo Green is noticeably darker, more muted, warmer than Royal Cloak.

Can I use Phthalo Green instead of Royal Cloak?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Phthalo Green is noticeably darker, more muted, warmer than Royal Cloak.

Can I use Royal Cloak instead of Phthalo Green?

The same difference applies the other way round: Royal Cloak is noticeably lighter, more saturated, 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.