In Lassaigne’s test, an organic compound produces a blood-red color with FeCl₃ but no violet color with sodium nitroprus
Blood-red color with FeCl₃ indicates nitrogen and sulphur together (NaSCN), while no violet color with sodium nitroprusside suggests sulphur alone isn’t detected, but the former confirms both.
Ref: NCERT Class 11 Chemistry > Chapter 8: Organic Chemistry - Some Basic Principles and Techniques > Topic: Classification and Nomenclature of Organic Compounds - IUPAC