Blue is the Warmest Color

Adele (Adèle Exarchopoulos) is a high school student who dates boys, at her friends' urging, but is unhappy. She sees a blue-haired woman on the street (Emma) and is immediately attracted to her.

One night she goes to a lesbian bar looking for her, and sees her. Emma (Léa Seydoux) approaches her, and it is the beginning of a passionate love affair that lasts for several years.  Emma is open about her sexuality but Adele hides hers from her conservative family, friends and later colleagues.

Emma is an artist and her paintings of Adele win her acclaim. As Emma grows artistically and intellectually she pushes Adele to as well, but Adele is comfortable being a nursery school teacher and has no wish to change. Distance grows between them and Adele impulsively has a few sexual encounters with a male colleague to assuage her loneliness. Emma finds out and throws Adele out of their house, telling her she never wants to see her again.

After time has passed the two meet again. Emma is now in a relationship with a woman with a young son. Adele is still alone and still in love with Emma, but Emma tells her that while she'll always have infinite tenderness for Adele her life is with someone else now. 
Emma has an exhibit of her work at a prestigious art gallery and Adele attends. She sees the paintings Emma has painted of her but Adele herself is largely ignored. She leaves the gallery, accepting that she is no longer a part of Emma's world.

Thanks Tiaga!