Deep Blue vs Royal Blue

How close are they?

ΔE 7.9, close: similar, with a visible difference side by side. Royal Blue is a little lighter, warmer than Deep Blue.

Can I use Royal Blue instead of Deep Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Royal Blue is a little lighter, warmer than Deep Blue.

Can I use Deep Blue instead of Royal Blue?

The same difference applies the other way round: Deep Blue is a little darker, 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.