Why are some red blood Cells darker than brighter Cells?

Answer 1

Mature red blood cells resemble a donut, with a sort of membrane in the center. Under a light microscope, the thinner center of the red blood cell allows more light through, and the transparency gives it a lighter shade.

Answer 2

Red blood cells are thicker on the edges and thinner in the center, consequently they look paler in the center when viewed under a microscope. This somewhat odd cross-sectional shape is beneficial to the cells and allows them to deform more easily to pass through capillaries that have a narrower diameter than the un-deformed red blood cell so that the cell can move through the capillary delivering its load of oxygen and picking up the waste CO2 from the cells along the capillary.

Answer 3

Oxygenated [arterial] haemoglobin is bright red while de-Oxygenated [venous] haemoglobin is a pale purple.

This difference depends upon the presence or absence of Oxygen.