ΔE 12.6, fair: the same colour family, but not a straight swap. Lime Green is cooler than Nurgle's Rot. They are different kinds of paint: Citadel Nurgle's Rot is a technical paint or medium and Vallejo Lime Green is an opaque base/layer paint, 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 13). Lime Green is cooler than Nurgle's Rot. They are different kinds of paint.
The same difference applies the other way round: Nurgle's Rot is warmer.
ΔE is the colour difference between our two swatch estimates: under 2 is a perfect match (you’d struggle to tell them apart), under 6 very close, under 10 close, under 15 fair, under 20 loose, under 30 very loose, and 30 or more a poor match. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Nurgle's Rot on the colour wheel See Lime Green on the colour wheel
All matches: Nurgle's Rot equivalents · Lime Green equivalents