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