How can you display water mark using a condition

Hi,

           I want to apply water mark to one text box using condition, Text box's are getting generating dynamically and has bind to a list all text box's use same data temple. My requirement is to set water mark only one text box based on condition. Please suggest some solution.If the list contains 4 key value pair four text box will get displayed

I'm new to wpf, Is there any way to change text dynamically using xaml,key is label and value is displayed in text box, only for one text box I have to show water mark.

Regards,

Shijith