Hexwraith Flame vs Hulky Green

How close are they?

ΔE 33.2, loose: different enough that you should pick on purpose. Hulky Green is noticeably darker, much more muted than Hexwraith Flame.

Can I use Hulky Green instead of Hexwraith Flame?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 33). Hulky Green is noticeably darker, much more muted than Hexwraith Flame.

Can I use Hexwraith Flame instead of Hulky Green?

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