Ultramarine Blue vs Phthalo Blue GS

How close are they?

ΔE 11.4, fair: the same colour family, but not a straight swap. Phthalo Blue GS is a little darker, more saturated, warmer than Ultramarine Blue.

Can I use Phthalo Blue GS instead of Ultramarine Blue?

Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Phthalo Blue GS is a little darker, more saturated, warmer than Ultramarine Blue.

Can I use Ultramarine Blue instead of Phthalo Blue GS?

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