Blue Flux vs Sky Blue

How close are they?

ΔE 9.1, close: similar, with a visible difference side by side. Sky Blue is a little darker, more muted than Blue Flux.

Can I use Sky Blue instead of Blue Flux?

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

Can I use Blue Flux instead of Sky Blue?

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

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.