Blue to violet

You: Well, the Biuret test uses sodium hydroxide and copper sulphate, which has a blue color. 

Bubble: The test derives its name from the compound Biuret, which is derived from urea and similarly to proteins, also contains the -CO-NH- group and gives a positive test. 

Monica: In the presence of dilute copper sulphate in alkaline solution, nitrogen atoms in the peptide chain form a violet complex with copper (II) ions (Cu2+).

Dr. Brown: Well done to both of you. It seems to me that you are ready to carry out the test. 

You perform the test using the steps below.

1. In a test tube, transfer 2 ml of the urine sample.

2. Add 3 ml of 40% sodium hydroxide and shake the test tube well. 

3. Add 5 drops of 1% copper sulphate.

4. Observe for any color change.

Website: https://microbenotes.com/testing-for-biological-molecules/