Question
I live in southwestern PA (south of pittsburgh).  I planted 4 shirobania spirea at my house and two at my mothers this spring.  I have three blooming solid white and one solide pink.  She has one blooming solid pink and one blooming white pink and dark pink (like they are supposed to).  Is there something these plants need to get the correct bloom color? they are all planted in full sun.

Answer
Windy,
This form of Spirea japonica is a hybrid and like most hybrid plants, sometimes there are genetic codes that are dominant. Hybrids can also be unpredictable, you could get pink flowers on the ones that bloomed white and white on the ones that bloomed pink or you could get a mixture of pink and white.
Prune the plants after the flowers fade to stimulate a second flush of flowering.
As far as doing anything, there is nothing that you can do to make certain that the plants flower the color you want them to.
I hope this helped.
Good luck.
Ed Gulliksen