As you see in this video, blood mostly reflects red light. Interestingly, though, blood also reflects a little bit of green light. If we shine white light (which contains all colors) onto the blood, blood looks red since it reflects so much more red light than green light. However, if we use a light source that contains all of the visible colors except red and shine it onto the blood, the blood will be green. With no red light present in the first place, the blood can't reflect any red light. The only thing left that it can reflect is the green light. The blood is therefore green. Note that this is not a trick of the eyes. The blood is literally green. In fact, human blood is always a little bit green. We usally don't notice the green color of blood because there is typically so much more red light being reflected by the blood. But if you shine a light on the blood that contains green light but no red light, the green color of blood becomes obvious. Water is naturally very slightly blue colored because it absorbs some of the red light passing through. The deeper you go in the ocean, the less red light there is in the sunlight that reaches you. Without red color in the sunlight, only green light reflects from the blood. This fact can be startling to divers who get a cut while diving. Again, the blood does not change when in the deep ocean. Rather, the green color of blood that is always there becomes obvious once the brighter red color is no longer present. Since the green reflectance peak of blood is always there, blood can be obviously green anytime you have a light source with no red color, and not just in the deep ocean. hwww.youtube.com/...h-i