ΔE 19.7, loose: different enough that you should pick on purpose. Beetle Green is a little lighter, more saturated, cooler than Forrest Flux.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Beetle Green is a little lighter, more saturated, cooler than Forrest Flux.
The same difference applies the other way round: Forrest Flux is a little darker, more muted, warmer.
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.
See Forrest Flux on the colour wheel See Beetle Green on the colour wheel
All matches: Forrest Flux equivalents · Beetle Green equivalents