Polychromasia refers to the presence of red blood cells that have a blue-gray tint to them, indicating that they are in the process of maturing. Polychromasia is often seen in medical conditions such as hemolytic anemia, megaloblastic anemia, and in the recovery phase following iron deficiency anemia or blood loss.

There is a strong correlation between polychromasia and reticulocytosis. Blue-gray erythrocytes are reticulocytes, but not all reticulocytes are necessarily blue-gray in color. Manual reticulocyte counting is possible with new methylene blue or brilliant cresyl blue, among others, but in modern practice it is often done with a hemocytometer with built-in staining.