Turnbull's blue is a .................

Ferrous ferricyanide compounds are also known as Turnbull's blue.

Turnbull's blue is a pigment used in ink and paint, It  is formed as a result of the reaction between  ${\mathrm{Fe}}^{2+}$ and potassium ferricyanide.

$3{\mathrm{Fe}}^{2+}+2{\mathrm{K}}_3\left[\mathrm{Fe}(\mathrm{CN}{)}_6\right]\to {\mathrm{Fe}}_3{\left[\mathrm{Fe}(\mathrm{CN}{)}_6\right]}_2\text{ (blue ppt.) }$