ΔE 13.1, fair: the same colour family, but not a straight swap. Off-White is a little darker, more saturated than Flat White.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Off-White is a little darker, more saturated than Flat White.
The same difference applies the other way round: Flat White is a little lighter, 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 Flat White on the colour wheel See Off-White on the colour wheel
All matches: Flat White equivalents · Off-White equivalents