There is no configurable attribute in FileMaker for this. It is indeed a OS setting, but the actual color you see is a function of the highlight color specified at the system level and the fill color (or background color) specified for that field. There's some kind of calculation that takes place that attempts to produce a color that will contrast with the colors present in the field at the time the text is selected.
> There's some kind of calculation that takes place
That seems to describe it. :-) Text color also plays a role, I can also confirm; "black" seems to be a safe bet, but it would be nice to see something in the FM documentation about this bu.. er, strange behavior, as it appears to be entirely unpredictable.
Sorry, I forgot to say thanks. The speedy response is greatly appreciated!