Water absorbs red wavelengths first, leaving mostly blue and green light to reflect off objects.
Because of this selective absorption, the natural red color of blood can’t be seen, and it takes on a greenish hue instead.
This color change is purely an optical illusion caused by the physics of light refraction and absorption, not an actual change in your blood’s color.
According to UCSB Science Line and IFLScience, your blood always remains red, but the underwater environment alters how it appears to your eyes. #fblifestyle