In thread702-647405 I describe a technique for highlighting a single row of a continuous form based on a condition.
In your case, I think you only want to highlight a single field, but you can use the same technique, but without the need for the background field. Simply apply the conditional formatting to the individual textbox that you wish to manipulate.
Good Luck
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As a circle of light increases so does the circumference of darkness around it. - Albert Einstein