Bright Violet vs Warpfiend Grey

How close are they?

ΔE 6.8, close: similar, with a visible difference side by side. Warpfiend Grey is a little lighter, more muted than Bright Violet.

Can I use Warpfiend Grey instead of Bright Violet?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Warpfiend Grey is a little lighter, more muted than Bright Violet.

Can I use Bright Violet instead of Warpfiend Grey?

The same difference applies the other way round: Bright Violet is a little darker, more saturated.

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.