Slaanesh Grey vs Smoked Blue

How close are they?

ΔE 15.3, loose: different enough that you should pick on purpose. Smoked Blue is noticeably darker, cooler than Slaanesh Grey.

Can I use Smoked Blue instead of Slaanesh Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Smoked Blue is noticeably darker, cooler than Slaanesh Grey.

Can I use Slaanesh Grey instead of Smoked Blue?

The same difference applies the other way round: Slaanesh Grey is noticeably lighter, warmer.

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.