Warpfiend Grey vs Violet

How close are they?

ΔE 18.4, loose: different enough that you should pick on purpose. Violet is noticeably darker, more saturated, warmer than Warpfiend Grey.

Can I use Violet instead of Warpfiend Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Violet is noticeably darker, more saturated, warmer than Warpfiend Grey.

Can I use Warpfiend Grey instead of Violet?

The same difference applies the other way round: Warpfiend Grey is noticeably lighter, more muted, cooler.

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.