A positive test is indicated by: a deep blue/purple color due to the copper ion complex with the amide group of the protein.
How does a biuret test work?
The biuret test measures peptide bonds in a sample. ... In an alkaline solution, copper II is able to form a complex with the peptide bonds. Once this complex has been formed, the solution turns from a blue color to a purple color. The deeper the purple color, the more peptide-copper complexes that have been formed.