In Lassaigne’s extract of an organic compound, both nitrogen and sulphur are present, which gives blood red colour with ${\text{Fe}}^{3+}$ due to the formation of:

When both Nitrogen and Sulphur are present in an organic compound, Lassaigne's test produces Sodium thiocyanate ($\text{NaSCN}$). When this extract is acidified and treated with Ferric chloride (${\text{FeCl}}_3$), it reacts to form a blood-red colored complex:
$${\text{Fe}}^{3+}+{\text{SCN}}^{-}\to [\text{Fe}(\text{SCN}){]}^{2+}$$
(Note: Some texts represent the complex as $[\text{Fe}(\text{SCN}{)}_3]$ or similar, but the ionic species responsible is as shown).