Ducat Blue vs Blue Flux

How close are they?

ΔE 9.4, close: similar, with a visible difference side by side. Blue Flux is a little darker, more saturated than Ducat Blue.

Can I use Blue Flux instead of Ducat Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Blue Flux is a little darker, more saturated than Ducat Blue.

Can I use Ducat Blue instead of Blue Flux?

The same difference applies the other way round: Ducat Blue is a little lighter, more muted.

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.