ΔE 10.3, fair: the same colour family, but not a straight swap. Slaanesh Grey is a little darker, cooler than Thanatos Grey.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 10). Slaanesh Grey is a little darker, cooler than Thanatos Grey.
The same difference applies the other way round: Thanatos Grey is a little 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.
See Thanatos Grey on the colour wheel See Slaanesh Grey on the colour wheel
All matches: Thanatos Grey equivalents · Slaanesh Grey equivalents