~Red milk.~ The most common trouble of this nature in milk is due to presence of blood, which is most frequently caused by some wound in the udder. The ingestion of certain plants as sedges and scouring rushes is also said to cause a bloody condition; madders impart a reddish tinge due to coloring matter absorbed. Defects of this class can be readily distinguished from those due to germ growth because they are apparent at time of milking. Where blood is actually present, the corpuscles settle out in a short time if left undisturbed. There are a number of chromogenic or color-producing bacteria that are able to grow in milk, but their action is so slow that generally they are not of much consequence. Moreover their development is usually confined to the surface of the milk as it stands in a vessel. The most important is the well-known _B. prodigiosus_. Another form found at times in milk possessing low acidity[73] is _B. lactis erythrogenes_. This species only develops the red color in the dark. In the light, it forms a yellow pigment. Various other organisms have been reported at different times.[74] ~Blue milk.~ Blue milk has been known for many years, its communicable nature being established as long ago as 1838. It appears on the surface of milk first as isolated particles of bluish or grey color, which later become confluent, the blue color increasing in intensity as the acidity increases. The causal organism, _B. cyanogenes_, is very resistant toward drying,[75] thus accounting for its persistence. In Mecklenberg an outbreak of this sort once continued for several years. It has frequently been observed in Europe in the past, but is not now so often reported. Occasional outbreaks have been reported in this country. ~Other kinds of colored milk.~ Two or three chromogenic forms producing still other colors have occasionally been found in milk. Adametz[76] discovered in a sample of cooked milk a peculiar form (_Bacillus synxanthus_) that produced a citron-yellow appearance which precipitated and finally rendered soluble the casein. Adametz, Conn, and List have described other species that confer tints of yellow on milk. Some of these are bright lemon, others orange, and some amber in color. Still other color-producing bacteria, such as those that produce violet or green changes in the milk, have been observed. In fact, almost any of the chromogenic bacteria are able to produce their color changes in milk as it is such an excellent food medium. Under ordinary conditions, these do not gain access to milk in sufficient numbers so that they modify the appearance of it except in occasional instances.