Necron Compound vs Dark Copper

How close are they?

ΔE 17.8, loose: different enough that you should pick on purpose. Dark Copper is a little darker, much more saturated than Necron Compound.

Can I use Dark Copper instead of Necron Compound?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Dark Copper is a little darker, much more saturated than Necron Compound.

Can I use Necron Compound instead of Dark Copper?

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