ΔE 6.5, close: similar, with a visible difference side by side. Ivory is a little lighter, more muted than Tombstone Yellow.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Ivory is a little lighter, more muted than Tombstone Yellow.
The same difference applies the other way round: Tombstone Yellow 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.
See Tombstone Yellow on the colour wheel See Ivory on the colour wheel
All matches: Tombstone Yellow equivalents · Ivory equivalents