Blood red colouration indicates the presence of nitrogen and sulfur in the Lassaigne solution.

Lassaigne Test
The Lassaigne test is a qualitative test used to detect the presence of nitrogen, sulfur, and halogens in an organic compound. It involves the fusion of the organic compound with sodium metal and subsequent extraction of the resulting compound with water. The resulting solution is known as Lassaigne solution, which can then be further tested for the presence of specific elements.

FeCl₃ Test
The addition of FeCl₃ solution to the acidified Lassaigne solution is a confirmatory test for the presence of nitrogen. FeCl₃ reacts with nitrogen to form a blood red complex, known as the Prussian blue complex. This coloration is characteristic and indicates the presence of nitrogen in the Lassaigne solution.

Explanation
When FeCl₃ solution is added to the acidified Lassaigne solution, the FeCl₃ reacts with the nitrogen compounds present in the solution to form the Prussian blue complex. This complex has a deep blood red coloration, which is easily visible.

The presence of nitrogen in the Lassaigne solution can be attributed to the presence of nitrogen-containing functional groups in the original organic compound. These functional groups may include amines, amides, nitro compounds, or other nitrogen-containing groups.

It is important to note that FeCl₃ does not react with sulfur directly. However, the presence of sulfur in the Lassaigne solution is necessary for the formation of the Prussian blue complex. Sulfur reacts with the nitrogen compounds to form sulfur-nitrogen compounds, which then react with FeCl₃ to form the colored complex.

Therefore, the blood red colouration produced upon addition of FeCl₃ solution to the acidified Lassaigne solution indicates the presence of nitrogen and sulfur in the original organic compound. The presence of nitrogen is confirmed by the formation of the Prussian blue complex, while the presence of sulfur is necessary for this reaction to occur.

Blood red colouration is due to formation of FeCNS.