Not sure actually. I'm really sure the first one is not real. I've never seen a only a vein before (only drawings, and they weren't in colour), but I believe they only exist in red, so I think it's the third option: light.
Ahh wow I watching stuff about this just like a few weeks ago. My teachers in the past in taught us that it's blue because of the lack of oxygen, but doesn't it actually have to do with light refraction?