Necron Compound vs Chaos Bronze

How close are they?

ΔE 18.9, loose: different enough that you should pick on purpose. Chaos Bronze is a little lighter, much more saturated than Necron Compound.

Can I use Chaos Bronze instead of Necron Compound?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Chaos Bronze is a little lighter, much more saturated than Necron Compound.

Can I use Necron Compound instead of Chaos Bronze?

The same difference applies the other way round: Necron Compound is a little darker, much 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.