I think ist is usefull, to make labels transparent without create an annotation . I need this usage very often.
Colors can be partially or wholly transparent. 100 percent transparent means they are fully transparent, or effectively invisible. A quick way to set a color to 100 percent transparent is to click No Color directly on the color picker instead of opening the Color Editor dialog box. (Note that there are still values assigned to the other color channels when you do this.) When colors are 0 percent transparent, they are completely opaque.