Ultramarines Blue vs Nightshade Purple

How close are they?

ΔE 25.4, loose: different enough that you should pick on purpose. Nightshade Purple is much more muted, warmer than Ultramarines Blue.

Can I use Nightshade Purple instead of Ultramarines Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Nightshade Purple is much more muted, warmer than Ultramarines Blue.

Can I use Ultramarines Blue instead of Nightshade Purple?

The same difference applies the other way round: Ultramarines Blue is much more saturated, 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.