Hexwraith Flame vs Troll Green

How close are they?

ΔE 26.4, loose: different enough that you should pick on purpose. Troll Green is noticeably darker, more muted than Hexwraith Flame.

Can I use Troll Green instead of Hexwraith Flame?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Troll Green is noticeably darker, more muted than Hexwraith Flame.

Can I use Hexwraith Flame instead of Troll Green?

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