RAF Azure Blue vs Blue Flux

How close are they?

ΔE 21.1, loose: different enough that you should pick on purpose. Blue Flux is much more saturated, cooler than RAF Azure Blue.

Can I use Blue Flux instead of RAF Azure Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Blue Flux is much more saturated, cooler than RAF Azure Blue.

Can I use RAF Azure Blue instead of Blue Flux?

The same difference applies the other way round: RAF Azure Blue is much more muted, 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.