Here's why:

* Biuret reagent is a solution of copper(II) sulfate in a strong base.

* Peptide bonds are the bonds that link amino acids together to form proteins and peptides.

* The reaction: When biuret reagent is added to a solution containing peptide bonds, the copper ions in the reagent react with the nitrogen atoms in the peptide bonds. This reaction creates a complex that absorbs light in the visible spectrum, resulting in the characteristic light purple color.

Therefore, a light purple color with biuret reagent is a positive test for the presence of proteins or peptides.