Gothic Blue vs Phthalo Blue GS

How close are they?

ΔE 11.9, fair: the same colour family, but not a straight swap. Phthalo Blue GS is a little darker, more saturated than Gothic Blue.

Can I use Phthalo Blue GS instead of Gothic Blue?

Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Phthalo Blue GS is a little darker, more saturated than Gothic Blue.

Can I use Gothic Blue instead of Phthalo Blue GS?

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

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.