It seems like that Color property ion the TPanel is a little bit buggy.
Sometimes, I can set the ParentColor := true and then set Color to e.g. clRed.
Then I can set the Panel background color, but sometimes it doesn't work...
I suggest you create a TShape in the TPanel and align it to alClient instead.
...Or you can create a component where you draw everything by yourself.
//Nordlund