Royal Blue vs Phthalo Blue (Green Shade)

How close are they?

ΔE 17.9, loose: different enough that you should pick on purpose. Phthalo Blue (Green Shade) is much more saturated, warmer than Royal Blue.

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

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Phthalo Blue (Green Shade) is much more saturated, warmer than Royal Blue.

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

The same difference applies the other way round: Royal Blue 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.