ΔE 13.8, fair: the same colour family, but not a straight swap. Flak Gun Yellow is noticeably lighter, more saturated than Basilisk Brown.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Flak Gun Yellow is noticeably lighter, more saturated than Basilisk Brown.
The same difference applies the other way round: Basilisk Brown is noticeably darker, more muted.
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 Basilisk Brown on the colour wheel See Flak Gun Yellow on the colour wheel
All matches: Basilisk Brown equivalents · Flak Gun Yellow equivalents