ΔE 14.8, fair: the same colour family, but not a straight swap. Flak Gun Yellow is noticeably darker, more muted than Imp Yellow.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 15). Flak Gun Yellow is noticeably darker, more muted than Imp Yellow.
The same difference applies the other way round: Imp Yellow 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.
See Imp Yellow on the colour wheel See Flak Gun Yellow on the colour wheel
All matches: Imp Yellow equivalents · Flak Gun Yellow equivalents