i never heard of color washing out if the inside of a flower petal .... with plain water ...

i have heard of water droplets magnifying sunlight and burning the tissue beneath white ... and then brown ...

ken

Petunias surely can get snotty looking after rains, and it's especially noticeable in the more vibrant ones like reds and deep blues. It's caused by botrytis, a fungal disease because petunia blooms, being large and fragile tend to collapse in on themselves for awhile after a rain and give the disease the conditions it needs to 'bloom'.

Some varieties are more tolerant of those conditions and don't go ugly and those would be the multiflora types. I know that I have had very good weather tolerance with the "dreams" series and they are the only variety I will grow in bedding petunias for reds and blues.

I've grown Surfina "Red" petunias in containers the past several years - they stand up to rainy weather better than any others I've tried. And they love, love, love hot weather.