Set "Loading" screen background color on Flex

Check out the compiler arguments line in the Flex properties and add
-default-background-color #PUTYOURCOLORHERE

Check the complete list compiler aurguments at :-
http://livedocs.adobe.com/flex/201/html/wwhelp/wwhimpl/common/html/wwhelp.htm?context=LiveDocs_Book_Parts&file=compilers_123_24.html