Demon Red vs Slaanesh Grey

How close are they?

ΔE 16.7, loose: different enough that you should pick on purpose. Slaanesh Grey is a little lighter, cooler than Demon Red.

Can I use Slaanesh Grey instead of Demon Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Slaanesh Grey is a little lighter, cooler than Demon Red.

Can I use Demon Red instead of Slaanesh Grey?

The same difference applies the other way round: Demon Red is a little darker, 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.