Viking Blue vs Royal Blue

How close are they?

ΔE 9.5, close: similar, with a visible difference side by side. Royal Blue is a little darker, more muted than Viking Blue.

Can I use Royal Blue instead of Viking Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Royal Blue is a little darker, more muted than Viking Blue.

Can I use Viking Blue instead of Royal Blue?

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