When I want to extend canvas size the extension background color is often (but not always) disabled; other times, is my last (default) selection, is given. If one of you could show me where/how this setting is made, I would be grateful.
The background color is used to extend the canvas when your document contains a background layer that is flattened without transparency. Look to the bottom of the layer stack. If the layer is named background with italicized font, a background color will get added to that layer when extending the canvas.
If all of the layers float, meaning no background layer, the extended canvas will be transparent. Transparency is represented with a light grey checkerboard.
The background color is nested with the foreground color at the bottom of the tools panel. The foreground color is the large color chip on top, and the background is the color chip under and slightly down and to the right.