Green and White Tara

Tara was born from the tears of compassion, it's said, of the bodhisattva Avalokiteshvara.
He saw the suffering of the people and his tears formed a lake, from which a lotus grew.
When the lotus opened the Goddess Tara was revealed.

The story goes that the Green Tara was born from the tears of his right eye, and the White Tara from the tears of his left eye.

Green Tara is often depicted in an half open lotus, symbolising the night.
She stands for virtuous activity.
White Tara is shown in a lotus in full bloom, often with folded legs in meditation and represents the day.
She’s pictured as being endowed with seven eyes (look at the palms of the hands, soles of the feet, and her forehead) to symbolize the watchfulness of the compassionate mind. She shows serenity, grace and peacefulness.

Together they represent the unending compassion of the Goddess who works day and night to end all suffering.

Ofcourse this is a simplification and there's a lot more to this difference, especially from different regions and traditions, but I think you get the idea.