The other tool you can use to quickly remove the background of simple shapes like a plate is the crop tool.
(NB Usually I'm able to select the circle shape from the menu, and adjust it so that it perfectly fits the object. It wouldn't let me do that on this photo for some reason, so I had to use the square shape. I sized the square by dragging the sides to the rim of the plate, then clicked the Ellipse shape so it looked like this - if you can see the fine, dotted line around the plate, that is).

After that, you just select Crop (right click on the photo).
The background will automatically be cut, and filled with the second colour choice. (There's two squares on the top right that show current colour choices - in the example below, white is colour 1, purple is colour 2).
This is what it looks like after a crop

The same method would work for any simple shape, so good for books, plates etc.
I reject your reality and substitute my own. ~Adam Savage