As far as I'm aware, the Windows API will mask the toolbar with the colour of the far bottom-right pixel. So, if this is magenta, then the rest of the icon strip that is magenta will be masked.
Make sure that this pixel is your background colour (i.e. you don't have part of an icon drawn there).
Edit: On second thoughts, it might be top left. :-)
Make sure that this pixel is your background colour (i.e. you don't have part of an icon drawn there).
Edit: On second thoughts, it might be top left. :-)