How does georgia o'keeffe use two colors that generally clash to create harmony in her painting two calla lilies on pink?

The correct answer should be - she adds white to the colors to create pastel versions.
If you take a look at the painting, you can see that the white color predominates in it - most of the painting is covered in white, whereas the background is light pink. She mixed these two colors in order to create a sense of pastel image, which is pleasing to the eye.