Warpfiend Grey vs Grey Blue

How close are they?

ΔE 17.2, loose: different enough that you should pick on purpose. Grey Blue is noticeably lighter, cooler than Warpfiend Grey.

Can I use Grey Blue instead of Warpfiend Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Grey Blue is noticeably lighter, cooler than Warpfiend Grey.

Can I use Warpfiend Grey instead of Grey Blue?

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