Seasonal change is unlikely to account for the difference in markings this time. Numerous other P. vigintimaculata adults are (somewhat) nearby, and they all have about the same proportion of white, which is more white than this specimen. This one was traveling alone, a few feet from the nearest known crowd of them--a population on a zucchini plant, a good source of mildew.

In the three major locations for 20-spots that I've found near my office, there is some significant color/pattern variation - two of the populations are white with black and orange markings, the other is tan with dark brown markings. Each population is separated by about 5 city blocks. Obviously that's much more widely-separated than the ones in your garden/yard, but these little beetles definitely do have very noticeable and very local variations. Maybe because they're so small and eat something that doesn't move - they don't have to fly around to find prey like the carnivorous beetles, and may not fly as strongly or as far because they're small and more affected by wind and weather. (The latter is a speculation, I'll see if I can find research on it.)

I just know that I've read comments here that shed some light on the possible causes of lady beetle color/pattern variations, but I am having a heck of a time re-finding them! This is the only one I've been able to dig-up thus far:

http://bugguide.net/node/view/6938#5076

I'll keep looking..