A review of how colors clue us into gross diagnosis in domestic animals.

Different tissues have a normal color spectrum that reflects their cellular composition and/or metabolic features. Similarly, distinct color variations may occur in tissues that have undergone pathologic or nonpathologic changes. Common examples of color changes in domestic animal tissues include red (associated with erythrocytes, hemoglobin, and myoglobin), brown (ferric hemoglobin or myoglobin, suppurative inflammation, lipid oxidation, postmortem autolysis, formalin fixation, neoplasms arising from cytochrome-rich tissues), yellow (hemoglobin and iron degradation, biliary pigment and by-products, carotenes, keratin, necrosis, suppurative or fibrinous inflammation), green (hemoglobin and iron degradation, biliary pigment and by-products, meconium, eosinophilic or suppurative inflammation, oomycete and algal infections), white (lack of blood, adipose tissue and its neoplasms, chylous effusion, necrosis, mineralization, fibrosis, lymphoid tissue, round cell neoplasms), translucent (transudate, cysts), black to gray (hemoglobin and iron degradation, melanin, carbon, tattoos), and blue to purple (poorly oxygenated blood, tattoos). Pathologists and pathology trainees can benefit from understanding why particular colors are present in a tissue or organ and are advised to recognize the color dynamics that occur over time, such as hemorrhage progressing from red to purple and subsequently to yellow, green, and brown. Therefore, clear and precise color recognition and description is a key feature of a gross examination. Understanding the relationship between color changes in tissues and the underlying biologic or pathologic processes can help elucidate disease recognition and diagnosis.