Xereus Purple vs Dark Ultramarine

How close are they?

ΔE 21.3, loose: different enough that you should pick on purpose. Dark Ultramarine is a little darker, more saturated, cooler than Xereus Purple.

Can I use Dark Ultramarine instead of Xereus Purple?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Dark Ultramarine is a little darker, more saturated, cooler than Xereus Purple.

Can I use Xereus Purple instead of Dark Ultramarine?

The same difference applies the other way round: Xereus Purple is a little lighter, more muted, warmer.

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.