Ultramarine Blue vs Phthalo Blue RS

How close are they?

ΔE 13.2, fair: the same colour family, but not a straight swap. Phthalo Blue RS is noticeably darker, warmer than Ultramarine Blue.

Can I use Phthalo Blue RS instead of Ultramarine Blue?

Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Phthalo Blue RS is noticeably darker, warmer than Ultramarine Blue.

Can I use Ultramarine Blue instead of Phthalo Blue RS?

The same difference applies the other way round: Ultramarine Blue is noticeably 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.