Based on the large leaf, you have a white mophead. But while the soil does not change the color of white blooms, many whites have colored parentage and exhibit some liiiiiiiittle areas in the blooms with color some years. Usually, the white center or white dot in the middle can turn a shade of pink, purple or blue. Other times, areas of the sepals do that instead. I have one Mme. Emile Mouillère that on some years (not always) does that (show colors in the center or in a few sepals). White blooms sometimes can also show some green or yellowish areas too... sometimes. I would not panic if you see this. Eventually, in the Summer or Fall, the spent bloom's sepals will start turning pinkish-greenish and finally end brown.

It is quite to rare to see them blooming at this time of the year though. Did you buy this one recently or are you south in Florida? My earliest blooming one is an unnamed lacecap that was supposed to be a Nikko Blue Hydrangea. But its blooms are in the broccoli phase, between 1/4" to 1" wide. The buds opened a month ago so originally, I thought it would be blooming by now but, looks like it stalled and will bloom mid-to-late April. Enjoy your early bloomage Christy; and add 3-4" of mulch if it does not have some already.

With a newly planted shrub like this, you may also be seeing some stress causing the flower to discolor as it dies. With different light and moisture levels from how it was grown previously, with perhaps a still different set of conditions in shipping or at the sales venue, a plant may lose flowers prematurely after planting, so you may not see this later in the season and in future years.

Thank you!