Check for a conditional format on the field in question. Enter layout mode, right click it and select the conditional format option to see if anything has been set up for it.
I think I know what you mean...
When you select the entire contents of a field, the field "highlights" with a particular colour, to indicate that you've selected it. This "highlight" or "selection" colour is sort of a complimentary colour, which FMP automatically varies depending on the colour of the text in the field and the fill colour you've set for the field.
As far as I know, there's no way to change this "highlight" or "selection" colour. You'll have to play around with your colour choices until you get a "highlight" colour that you're happy with.