ΔE 11.4, fair: the same colour family, but not a straight swap. Ivory is a little darker, more muted, warmer than Sky.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Ivory is a little darker, more muted, warmer than Sky.
The same difference applies the other way round: Sky is a little lighter, more saturated, cooler.
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 Sky on the colour wheel See Ivory on the colour wheel
All matches: Sky equivalents · Ivory equivalents