This chapter discusses reactions of proteins, which produce colors in certain reactions. Factors which neutralize the charge or remove the hydration shell precipitate the proteins from solution. If a strongly alkaline solution of a protein is treated with a few drops of dilute copper sulphate, a purple violet color is obtained. Albumins are simple native proteins present in blood, egg white and leguminous seed. Albumins are soluble in salt-free water. Globulins are also simple proteins present in blood, muscle and other animal tissues and pulses. Collagen is found in white fibrous tissue and organic substances in bone. Peptones are poorly defined intermediate products formed during the digestion of proteins by the proteolytic enzymes, as well as in the decomposition of proteins by hydrolysis and the putrefaction of proteins through the action of bacteria. Peptones are not precipitated by ammonium sulphate or coagulated by heat. Casein is the chief protein of milk.