Slaanesh Grey vs Sombre Grey

How close are they?

ΔE 8.9, close: similar, with a visible difference side by side. Sombre Grey is cooler than Slaanesh Grey.

Can I use Sombre Grey instead of Slaanesh Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Sombre Grey is cooler than Slaanesh Grey.

Can I use Slaanesh Grey instead of Sombre Grey?

The same difference applies the other way round: Slaanesh Grey is 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.