Grimdark Shadow vs Aether Blue

How close are they?

ΔE 41.3, loose: different enough that you should pick on purpose. Aether Blue is noticeably lighter, much more saturated than Grimdark Shadow.

Can I use Aether Blue instead of Grimdark Shadow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 41). Aether Blue is noticeably lighter, much more saturated than Grimdark Shadow.

Can I use Grimdark Shadow instead of Aether Blue?

The same difference applies the other way round: Grimdark Shadow is noticeably 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.