What causes green veins to appear under the foot? (Photo)

Thank you for the question! To provide a precise and personalized assessment of your situation, I'd recommend scheduling an in-person exam to evaluate for possible venous insufficiency. Judging by the photo, your veins may in fact be normal or you may have clusters of small spider veins.  The latter can be caused by age, pregnancy, obesity, family history, or sedentary lifestyle (sitting or standing for long periods of time). Your discomfort may potentially be alleviated by wearing compression stockings or it may require a treatment such as sclerotherapy. 
An in-person exam with a board-certified plastic surgeon is the best way to assess your needs and obtain expert medical advice. Best of luck!

I believe that this is the inner ankle and the image shows the bottom of the foot on the upper or top of the image. These are spider/reticular complexes. Wear compression stockings. 

A full venous reflux evaluation is recommended as an initial step which includes an ultrasound scan of the lower extremities to determine if venous insufficiency or reflux is present. In essence, healthy leg veins contain valves that open and close to help the blood return back to the heart. On the other hand, venous reflux disease develops when the valves that keep blood flowing out of the legs and back to the heart no longer function, causing to pool in the legs and leading to signs and symptoms such as varicose veins, leg or ankle swelling, leg heaviness and fatigue, leg pain, aching or cramping, burning or itching of the skin, restless legs, skin changes or rashes, ulcers, open wounds, or sores. Factors that contribute to the presence of venous reflux include age, gender, family history, heavy lifting, multiple pregnancies, obesity, and prolonged standing. If venous reflux test is positive, your physician can help you make informed healthcare decision to manage you care.
Best, 

From the posted picture it is impossible to say what your problem is.  Foot veins are very common and can be normal. The posted veins are very small and the dark spots are unclear as to their source.  See a vein specialist for an evaluation.

The picture appears to show small reticular veins (1-3 mm diameter) on the instep of you foot, which is common. It suggests that there may be a problem with too much pressure in the veins of your leg due to a vein valve problem. The key to proper treatment is the correct diagnosis of the root cause of the vein problems of the leg. We start with a careful history and physical examination to identify risk factors along with the signs and symptoms of venous disorders. The next step is the use of noninvasive ultrasound diagnostics to create a road-map of the venous system to identify and locate valve dysfunction. Only then can an appropriate treatment plan be developed. If ultrasound evaluation identifies that the valves of the saphenous vein or perforator veins are not functioning normally, a minimally invasive catheter based treatment can be used. Radiofrequency ablation, the Venefit procedure, is my technique of choice for the treatment of valve dysfunction resulting in reflux of the long and short saphenous veins along with the perforator veins. This treatment is covered by most health insurance plans! The smaller veins can be treated with either laser or sclerotherapy. Sclerotherapy would be my first choice here.