ΔE 7.3, close: similar, with a visible difference side by side. Nurgle's Rot is a little darker than Light Green. They are different kinds of paint: AMMO by Mig Light Green is an ink, glaze or transparent 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.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Nurgle's Rot is a little darker than Light Green. They are different kinds of paint.
The same difference applies the other way round: Light Green is a little lighter.
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 Light Green on the colour wheel See Nurgle's Rot on the colour wheel
All matches: Light Green equivalents · Nurgle's Rot equivalents