Gothic Blue vs Phthalo Blue RS

How close are they?

ΔE 8.4, close: similar, with a visible difference side by side. Phthalo Blue RS is a little darker, warmer than Gothic Blue.

Can I use Phthalo Blue RS instead of Gothic Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Phthalo Blue RS is a little darker, warmer than Gothic Blue.

Can I use Gothic Blue instead of Phthalo Blue RS?

The same difference applies the other way round: Gothic Blue is a little lighter, 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.