In late June, around the time of the summer solstice, nearly all the wildflowers in our area of North Central Minnesota are white. This seems unusual because other times of the year I see colorful wildflowers. May and early June have yellow and pink wildflowers, while in July through October you see magenta, yellow, and purple. The very first wildflowers of the spring are on blooming shrubs, and they are also predominantly white.
From quick online research I learned that bright white flowers are more visible to a wider range of pollinators. In particular, white attracts flies better than other colors, while bees are attracted to both white and colors. So, the prevalence of white flowers in earliest spring is understandable because there is likely more flies than bees at that time. The late June dominance of white wildflowers is not as easily explained. It could be coincidental, or maybe there is a subtle relation to available pollinators.
Bouncing Bet
Oxeye Daisy
Yarrow
Hoary Alyssum
Harebell
Of course, there is an exception. Tiny purple harebells also bloom in late June, but they are very inconspicuous.
North MN Outdoors 
I have seen yarrow in other northern areas and have always thought them so lovely, although many people call them weed flowers.
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