Just like a banana, they too go from green to yellow to black when most ripe. But these days you find the plantain skin yellow with black spots and yet not fully ripe. I have experienced such cases where the skin is black with yellow spots which are a sign that it is ripe but yet when you peel, you got yourself a not very ripe enough plantain so they are not very sweet either. So these days even if the plantains are black and yellow, I go with the technique of pressing to see if they give in, just like you do for avocados. Lightly press the fruit and see if it is soft and use it accordingly. Store them at room temperature in a basket without any plastic bags to make them last longer.