Lately, I have had a number of reports that USGS color aerials are working sporadically, or not at all.
It's highly dependent on the time of day. I believe the servers at USGS are not down, but they do get overloaded.As of now, 9:44 am PST on August 15th, they seem to be working.
If you've gotten blank tiles in the past, you can clear them up by marking a rectangle and using the recycle button.
I will find out if there is anything I can do, other than wait for it to get more stable.