Solanine Green! It's made of potatoes!

Why no green potatoes? The green color is actually the chlorophyll, a natural plant pigment that is actually tasteless and harmless.
However, the green is a kind of warning because solanine is a potentially toxic alkaloid that develops in the same area as chlorophyll. So, potatoes with green are often higher in that alkaloid than normal potatoes. The bitter taste is the solanine. How much green does not determine the amount of solanine present because greenness differs for many reasons.

Why might potatoes be green? Light exposure is the most common cause - but age, temperature and variety can all be reasons. So, harvested too young? There will be some greening. Naturally thin skinned? There might be some greening. Therefore, some greening will happen without the threat of solanine poisoning.

If you are concerned still, peel the potato. The concentration of the alkaloid is greatest right below the peel. COoking or steaming them in water to at least 300 degrees will also help reduce the solanine present in the potato. Or throw it away and start over.