system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: So if blood isn't blue inside the veins because of lack of oxygen then why is it that veins seen through the skin sometimes appear blue?
gpt
Short version: Skin absorbs blue light ... then the veins reflect it.

Full explanation: Subcutaneous fat allows only blue light to penetrate skin all the way to veins, so this is the color that is reflected back. Less energetic, warmer colors are absorbed by skin before they can travel that far. Blood also absorbs light, so blood vessels appear dark. Arteries have muscular walls, rather than thin walls like veins, but they likely would appear the same color if they were visible through the skin.