Imperium Blue vs Royal Blue

How close are they?

ΔE 20.6, loose: different enough that you should pick on purpose. Royal Blue is a little lighter, much more muted, cooler than Imperium Blue.

Can I use Royal Blue instead of Imperium Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Royal Blue is a little lighter, much more muted, cooler than Imperium Blue.

Can I use Imperium Blue instead of Royal Blue?

The same difference applies the other way round: Imperium Blue is a little darker, much more saturated, 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.