Ultramar Blue vs Royal Blue

How close are they?

ΔE 8.2, close: similar, with a visible difference side by side. Royal Blue is more saturated, warmer than Ultramar Blue.

Can I use Royal Blue instead of Ultramar Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Royal Blue is more saturated, warmer than Ultramar Blue.

Can I use Ultramar Blue instead of Royal Blue?

The same difference applies the other way round: Ultramar Blue is more muted, 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.