The Colloidal Iron kit is used for visualization of carboxylated and sulphated groups of acid mucins and proteoglycans. This method is based on the principle of binding positively charged ferric ions (Fe3+) to negatively charged endings of acid mucopolysaccharides and proteoglycans. Excessive reagents are rinsed while the bound ferric ions get visualized using the Prussian Blue reaction. In this reaction potassium ferrocyanide causes light blue precipitations of iron ferrocyanide to appear. Finally, the sections are exposed to Van Gieson stain that selectively stains different tissue structures and in turn creates clear and visually rich contrast. The method can be combined with the PAS method; that way glycogen and neutral mucopolysaccharides would get differentially stained characteristically magenta.