ΔE 14.8, fair: the same colour family, but not a straight swap. Heavy Metal is noticeably darker than Necron Compound.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 15). Heavy Metal is noticeably darker than Necron Compound.
The same difference applies the other way round: Necron Compound is noticeably lighter.
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.
See Necron Compound on the colour wheel See Heavy Metal on the colour wheel
All matches: Necron Compound equivalents · Heavy Metal equivalents