(new day => new bunch of questions

I recently realized that - under MacOSX - the "BackDrop" looks different than under WinXP.
To be more precise: I 'set the BackDrop to the id of image "Texture"' and use the same image as background texture for my stack. Under WinXP, both backdrop and stack thus look the same, while under MacOSX the backdrop looks bluish (i.e., the backdrop looks as if a modified copy of my "texture" had been used)
The look of the window indicates that the texture itself has been loaded properly.
Does anybody have an idea for this behaviour? Is it "a bug or a feature"?