Biuret test works for any compound containing two or more of these groups: CO-NH, CO-NH2, CH2-NH2, CNH-NH2, CS-NH2. Amino acids only have one.
Found this place in my own personal quest for some. There might be better. http://www.sciencecompany.com/sci-exper/food_chemistry.htm#Biuret%20Reagent
The copper atoms of Biuret solution (CuSO4 and KOH) will react with peptide bonds, producing a color change. A deep violet or blue color indicates the presence of proteins and a light pink color...
Purple / lavendar