Yellowed Bone vs Ivory

How close are they?

ΔE 7.8, close: similar, with a visible difference side by side. Ivory is a little darker, more muted than Yellowed Bone.

Can I use Ivory instead of Yellowed Bone?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Ivory is a little darker, more muted than Yellowed Bone.

Can I use Yellowed Bone instead of Ivory?

The same difference applies the other way round: Yellowed Bone is a little lighter, 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.