This doesn't appear to be a question concerned with FileMaker Server. Had you asked it in FM PRo Forum (See tabs at top of screen), you would have received a question much sooner than now.
- Click the field while in layout mode
- Use the formatting tool bar or the Appearance tab in the inspector to select white for its text color.
- Right click the field
- Select "conditional format"
- Specify Self > 100
- Specify a text color of Red.
- Click Ok to dismiss the conditonal formatting dialog box.
If you want the text to be white in order to make it invisible, I suggest using a conditional format expression to change the text's font size to 500 in order to make the contents of the field disappear instead.