ΔE 37.8, loose: different enough that you should pick on purpose. Aztec Turquoise Wash is noticeably lighter than Grimdark Shadow.
Can I use Aztec Turquoise Wash instead of Grimdark Shadow?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 38). Aztec Turquoise Wash is noticeably lighter than Grimdark Shadow.
Can I use Grimdark Shadow instead of Aztec Turquoise Wash?
The same difference applies the other way round: Grimdark Shadow is noticeably darker.
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.