What are the causes of big green vain in hands?

Normal: Normal hands often have visible vessels simply because the layer of tissue under the skin (the subcutaneous layer) is thinner over the hands so the veins are more shallow. Vein blood is mostly very dark red but the skin absorbs some colors better than others and the greenish tint is what your eye sees. Skin gets thinner with age, so veins often become more visible (and darker) over time.

Aging: Veins are normal blood vessels. They can look very big and green especially as we get older and our skin becomes thinner.