For those of us still using Flex Builder 3, every time we create a new project, we usually need to right-click the Project and add a parameter called -default-background-color=0xffffff into the compiler settings. An easier solution is to create an XML file with a name the same as your MXML file. So for example, if your Flex project is titled barnacles.mxml, then place this config file in the same directory and name it barnacles-config.mxml.