I have used a number of different components in a site all of which have their backgroud colour set to grey via actionscripted styles. The movie also has the same grey colour for its background. However when the components first load onto the screen they appear as a white box for a split second.
I have tried adding UI components bounding box to the library and changing its colour however this doesn't help.
if it is in with an html page, I have heard of other problems of people see a white box flash before the movie plays. Is that the problem your having? Like here is a white flashHERE
and if you are changing the size or position of them through code, set their visible property to false first , then sizeTo or move, then set visible to true.