The most significant factor is temperature. The recent heat wave has caused the bloom to have more white and less blue (July 27). My front yard is much cooler than the back and you can see the difference in the bloom color pattern.
This is what the blooms looked like before the hot days and warm nights (June 25)
Excessive heat may also cause irregular patterns or loss of pattern completely. Some blooms on the same stem pop out a completely different look.
It is normal for bicolor petunias to change color and/or pattern in different temperatures. You will find the stronger colors with cooler temperatures. The more direct sunlight, the more flowers. The older the bloom, the more faded the petals (Shockwave Denim is a great example of age-faded blooms).