During combination of parents DNA there is possibility
a) genes of one parent is dominant over other
b) both are resessive
c) both are dominant, and will not affect each other to being displayed as trait and thus appears together. These are known as Codominant
For example colour of eyes of an offspring having intermeditate colour of light brown and black. So in case of flowers as the flower color is codominant trait there are following possibilities
Some flowers with dark blue petals and some with white petals