Phthalo Blue (Green Shade) vs Royal Blue

How close are they?

ΔE 23.3, loose: different enough that you should pick on purpose. Royal Blue is much more muted, cooler than Phthalo Blue (Green Shade).

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

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Royal Blue is much more muted, cooler than Phthalo Blue (Green Shade).

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

The same difference applies the other way round: Phthalo Blue (Green Shade) is 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.