I learned quite a few things along the way during my WP7 app creation and submission.  Here is one problem (turned out to be a bug) that caused wasted time.

Due to a bug when icons are displayed in the program list and on the Start page (Tile), the tile icons displays properly showing the background theme color but on the program list the theme color is not shown and a grey background is shown instead: This doesn’t apply if you don’t have a transparent background for your icon. For example: