Marine Blue vs Royal Blue

How close are they?

ΔE 17.1, loose: different enough that you should pick on purpose. Royal Blue is a little lighter, much more muted, cooler than Marine Blue.

Can I use Royal Blue instead of Marine Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Royal Blue is a little lighter, much more muted, cooler than Marine Blue.

Can I use Marine Blue instead of Royal Blue?

The same difference applies the other way round: Marine 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.