While I am sure there is a fancy Javascript that you could use, that is a bit beyond my skills; however you can use the Action Builder for that; it just will take a few more seconds. Go to the Action Builder, create a new Condition, select your drop down box and then from the selection choose "text selected" and select the the drop down text you wish to have highlighted. In the Result section choose "Set the foreground color", select your drop down box, and then select the color you wish the text to be. Then if you want to have it removed if a user makes the selection, but it is an accident, you can create additional Actions where the foreground color is set back to say black when the other texts are selected. Not the most elegant way, but if you do not know Javascript it will at least get you there.
