Hexwraith Flame vs Green Ghost

How close are they?

ΔE 27.1, loose: different enough that you should pick on purpose. Green Ghost is noticeably darker, more muted, warmer than Hexwraith Flame.

Can I use Green Ghost instead of Hexwraith Flame?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Green Ghost is noticeably darker, more muted, warmer than Hexwraith Flame.

Can I use Hexwraith Flame instead of Green Ghost?

The same difference applies the other way round: Hexwraith Flame is noticeably lighter, more saturated, 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.