Gravelord Grey vs Stormfiend

How close are they?

ΔE 18.9, loose: different enough that you should pick on purpose. Stormfiend is a little lighter, much more saturated than Gravelord Grey.

Can I use Stormfiend instead of Gravelord Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Stormfiend is a little lighter, much more saturated than Gravelord Grey.

Can I use Gravelord Grey instead of Stormfiend?

The same difference applies the other way round: Gravelord Grey is a little darker, much more muted.

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.