Which one of the following complexes is violet in colour?

The colour in coordination complexes arises from d-d transitions, where the energy difference $$\Delta$$ between the split d-orbitals corresponds to the wavelength of light absorbed by the complex.

The sodium nitroprusside test involves the complex $$[Fe(CN)_5NO]^{2-}$$ reacting with $$S^{2-}$$ ions. The test reaction is:
$$[Fe(CN)_5NO]^{2-} + S^{2-} \;\longrightarrow\; [Fe(CN)_5NOS]^{4-} \quad-(1)$$

In reaction $$(1)$$, the nitrosyl ligand $$NO$$ is converted into a thionitrosyl ligand $$NOS$$ by the addition of the sulfide ion.

The product, $$[Fe(CN)_5NOS]^{4-}$$, exhibits a violet colour due to its characteristic d-d transition energy.

Comparing the given complexes:
A. $$Fe_4[Fe(CN)_6]_3\cdot H_2O$$ (Prussian blue, blue)
B. $$[Fe(CN)_5NOS]^{4-}$$ (violet)
C. $$[Fe(SCN)_6]^{4-}$$ (deep red)
D. $$[Fe(CN)_6]^{4-}$$ (pale yellow)

Therefore, the complex that is violet in colour is Option B: $$[Fe(CN)_5NOS]^{4-}$$.