Viking Blue vs Ultramarine Blue

How close are they?

ΔE 16.1, loose: different enough that you should pick on purpose. Ultramarine Blue is noticeably lighter, more muted than Viking Blue.

Can I use Ultramarine Blue instead of Viking Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Ultramarine Blue is noticeably lighter, more muted than Viking Blue.

Can I use Viking Blue instead of Ultramarine Blue?

The same difference applies the other way round: Viking Blue is noticeably darker, more saturated.

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.