Deep Blue vs Dark Blue

How close are they?

ΔE 9.9, close: similar, with a visible difference side by side. Dark Blue is more muted, cooler than Deep Blue.

Can I use Dark Blue instead of Deep Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Dark Blue is more muted, cooler than Deep Blue.

Can I use Deep Blue instead of Dark Blue?

The same difference applies the other way round: Deep Blue is 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.