I am fairly new to gimp (so forgive it this is a silly question) and am very frustrated with trying to erase or cut out part of a top layer only to reveal the background color (white) rather than the layer below. When I reduce the opacity of the top layer it reveals the one below. It is as if the background color is attached to the top layer! I was in a gimp workshop this week and the instructor was using the windows version and it worked perfectly. Perhaps there is a mac trick I need to know about. Thanks in advance for any help you can give me!