ΔE 27.4, loose: different enough that you should pick on purpose. Necron Compound is noticeably darker than Aluminum.
Can I use Necron Compound instead of Aluminum?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Necron Compound is noticeably darker than Aluminum.
Can I use Aluminum instead of Necron Compound?
The same difference applies the other way round: Aluminum 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.