What special property does Benedict's/Biuret's/iodine chemicals have?

each agent reacts with one of those substances. i.e. benedict's turns several different colors depending on the concentraion of sugar, but stays blue in the presence of starch. the iodine will turn any variation of dark brown to black with the presences of starch and starch only. the biuret should turn purple with the presence of protein and pinkish-purple with presence of peptides. theres no special property its just the way these chemicals react to those substances