ΔE 11.7, fair: the same colour family, but not a straight swap. Nurgle's Rot is more muted than Olive Green. They are different kinds of paint: AMMO by Mig Olive Green is an opaque base/layer paint and Citadel Nurgle's Rot is a technical paint or medium, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Nurgle's Rot is more muted than Olive Green. They are different kinds of paint.
The same difference applies the other way round: Olive Green is 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 Olive Green on the colour wheel See Nurgle's Rot on the colour wheel
All matches: Olive Green equivalents · Nurgle's Rot equivalents