Necron Compound vs Motherlode

How close are they?

ΔE 27.9, loose: different enough that you should pick on purpose. Motherlode is noticeably lighter, more saturated than Necron Compound.

Can I use Motherlode instead of Necron Compound?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Motherlode is noticeably lighter, more saturated than Necron Compound.

Can I use Necron Compound instead of Motherlode?

The same difference applies the other way round: Necron Compound is noticeably darker, 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.