Colorimetric assays use reagents that undergo a measurable color change in the presence of the analyte. They are widely used in biochemistry to test for the presence of enzymes, specific compounds, antibodies, hormones and many more analytes. For example, 
*para-Nitrophenylphosphate is converted into a yellow product by alkaline phosphatase enzyme. 
*Coomassie Blue is an aromatic dye that binds to aromatic proteins and positively charged amino acid residues within the protein structure. The binding interaction results in a spectrum shift, enabling quantitative measurement of the protein concentration. A similar colorimetric assay, the Bicinchoninic acid assay, uses a chemical reaction to determine protein concentration.
*The Biuret assay utilizes a biuret reagent which turns purple in the presence of proteins due to the chelation of copper salts in an alkaline solution.
*Enzyme linked immunoassays use enzyme-complexed-antibodies to detect antigens. Binding of the antibody is often inferred from the color change of reagents such as TMB.