A widely shared myth is that blood is blue until it is exposed to air or replenishes its oxygen.
Because veins are a greenish blue, that theory sounds reasonable enough.
But the fact is, human blood looks the same in your body as outside: red.
That hue is brighter when it’s oxygen-rich and darker when it needs that oxygen replenished, but it’s red all the same.
The tissue covering your veins affects how the light is absorbed and scattered, which is why the blood circulating your body looks blue.