ΔE 34.5, loose: different enough that you should pick on purpose. Septic Green is noticeably darker, much more muted, cooler than Flash Gitz Yellow.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 34). Septic Green is noticeably darker, much more muted, cooler than Flash Gitz Yellow.
The same difference applies the other way round: Flash Gitz Yellow is noticeably lighter, much more saturated, 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 Flash Gitz Yellow on the colour wheel See Septic Green on the colour wheel
All matches: Flash Gitz Yellow equivalents · Septic Green equivalents