Colour of K3[Fe(CN)6]
K3[Fe(CN)6] is a coordination compound consisting of potassium ions and a complex ion formed by a central iron atom surrounded by six cyanide ligands. The colour of the compound is a crucial characteristic that can be explained based on the nature of the ligands and the central metal ion.

Transition Metal Complex
Transition metal complexes like K3[Fe(CN)6] exhibit a variety of colours due to the d-d transitions that occur within the metal ion. These transitions involve the movement of electrons between different d orbitals of the transition metal ion, resulting in the absorption of specific wavelengths of light and the reflection of others.

Crystal Field Theory
In the crystal field theory, the ligands surrounding the metal ion create a crystal field that splits the d orbitals into different energy levels. The energy difference between these levels determines the colour of the complex. In the case of K3[Fe(CN)6], the cyanide ligands are strong-field ligands that cause a large splitting of the d orbitals, leading to a specific colour.

Colour of K3[Fe(CN)6]
The colour of K3[Fe(CN)6] is commonly described as deep red or blood red. This colour arises due to the absorption of light in the visible spectrum caused by the d-d transitions in the iron(III) ion within the complex. The specific shade of red can be attributed to the energy difference between the d orbitals in the presence of the cyanide ligands.
In conclusion, the deep red colour of K3[Fe(CN)6] is a result of the complex interplay between the transition metal ion, the ligands, and the crystal field theory. Understanding these factors is essential for predicting and explaining the colours of various transition metal complexes.