Green Hell vs Bright Green

How close are they?

ΔE 23.3, loose: different enough that you should pick on purpose. Bright Green is a little lighter, more saturated, warmer than Green Hell.

Can I use Bright Green instead of Green Hell?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Bright Green is a little lighter, more saturated, warmer than Green Hell.

Can I use Green Hell instead of Bright Green?

The same difference applies the other way round: Green Hell is a little darker, 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.