Question

Why are the veins on my legs and thighs dark green?

Answer 1

Because you have varicose veins.

Explanation:

They are called Varicose veins. Basically that means the blood has pooled and coagulated. It's a small form of embolism. They may be just a cosmetic issue, but might indicate a bigger problem. You should have a doctor look at them.

Here's the Wikipedia article if you want to read more:
https://en.wikipedia.org/wiki/Varicose_veins