ΔE 7.4, close: similar, with a visible difference side by side. Ivory is a little darker, more saturated, warmer than Rock Grey.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Ivory is a little darker, more saturated, warmer than Rock Grey.
The same difference applies the other way round: Rock Grey is a little lighter, more muted, 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 Rock Grey on the colour wheel See Ivory on the colour wheel
All matches: Rock Grey equivalents · Ivory equivalents