Final answer:
Hypochromasia is the abnormality that causes erythrocytes to appear lighter in colour due to decreased hemoglobin content, compared to normochromic which is normal, polychromasia indicating variation, and hyperchromatophilic suggesting darker staining.
Step-by-step explanation:
The abnormality in an erythrocyte that would exhibit a lighter-than-normal colour within the cell is known as hypochromasia. This condition indicates that there is a reduced amount of haemoglobin within the red blood cells (RBCs), which is why the cells appear paler. Normochromasia refers to normal colouring, polychromasia indicates a variation in colour due to the presence of immature red blood cells like reticulocytes, and hyperchromatophilic suggests darker staining, which is not related to paleness.