In reply to Sean Kelly:
Any greening could be harmful (even if cooked) as it indicates that there may be poisonous substances that could have formed within the tuber so best never to use. Bit of russian roulette iirc as it depends on quite a few variables. Best to be safe and never use any green potatoes.
If greening is extensive, dark in colour, or more than skin deep, I’ve always thrown the whole away (compost bin). However, if it’s small only surface (skin) and very light greening I’ve often just cut out the green part and cooked the tuber otherwise well. I’m not suggesting anyone should try this though. Individual sensitivity is variable. A btw I actually developed a skin sensitivity to the growing potato plant (sap) long time ago; same with tomato plants!