Transparency on Rich Texts & Rectangles makes their text translucent in editing canvas :-), but not so for Labels :-(
Setting the transparency slider (in the "background" section of the properties editor) for Rich Text and Rectangle widgets makes the contents of the widget correspondingly translucent in the editing canvas. I like this, because I can see what my page is going to look like without having to run a simulation.
Setting transparency for a Label widget doesn't work like that in the editing canvas.
I understand that the canvas is not 100% WYSIWYG (e.g., drop shadows), but this one seems more like a bug than a feature :-).