Dark Ivory vs White Grey

How close are they?

ΔE 7.2, close: similar, with a visible difference side by side. White Grey is a little lighter, more muted than Dark Ivory.

Can I use White Grey instead of Dark Ivory?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). White Grey is a little lighter, more muted than Dark Ivory.

Can I use Dark Ivory instead of White Grey?

The same difference applies the other way round: Dark Ivory is a little darker, more saturated.

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.