The original biuret solution is a nice blue color. When you add that to a solution without protein, you're basically just diluting the reagent, so it should be light blue. Depending on the solution, the copper might develop a sort of greenish, so green/light blue sounds good for the negative control reaction.

The original biuret solution is a nice blue color. When you add that to a solution without protein, you're basically just diluting the reagent, so it should be light blue. Depending on the solution, the copper might develop a sort of greenish, so green/light blue sounds good for the negative control reaction.

It is blue

Blue .
It turns purple in the presence of protein and pink in the presence of short polypeptide chains.
Biuret solution is a mixture of copper sulfate and potassium hydroxide. In the presence of a peptide chain, four of the N atoms in the peptide bonds of 2 chains will form a complex with the Cu(II) ion in solution. This complexation causes a change in the electronical properties of the Cu ion, and results in the observed color change. This color change is dependant on the number of formed complexes, so longer chains will have a more pronounced color change.