What part and at what stage of maturity?

The color of a banana is yellow.

Bananas are green... banannas are yellow, some bananas are red (we have them at the local markets), when they are very ripe they turn black.

Bananas are shipped green and go through a gassing process to start the ripening stage when they arrive in the US.
Go to your local grocery store and ask them about banana "stages" and they should be able to answer any questions and the many different types of bananas as some are for cooking/recipes more than eating 'raw'.
Hope this helps.