Thanatos Grey vs Warpfiend Grey

How close are they?

ΔE 15.5, loose: different enough that you should pick on purpose. Warpfiend Grey is a little darker, cooler than Thanatos Grey.

Can I use Warpfiend Grey instead of Thanatos Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Warpfiend Grey is a little darker, cooler than Thanatos Grey.

Can I use Thanatos Grey instead of Warpfiend 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.