ΔE 10.7, fair: the same colour family, but not a straight swap. Ivory is a little lighter, cooler than Tan (USAF).
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Ivory is a little lighter, cooler than Tan (USAF).
The same difference applies the other way round: Tan (USAF) is a little darker, 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 Tan (USAF) on the colour wheel See Ivory on the colour wheel
All matches: Tan (USAF) equivalents · Ivory equivalents