function onUpdate(dc) {
// determine background color and clear screen
var bgColor = getBackgroundColor();
var fgColor = (bgColor == Gfx.COLOR_BLACK) ? Gfx.COLOR_WHITE : Gfx.COLOR_BLACK;
dc.setColor(Gfx.COLOR_TRANSPARENT, bgColor);
dc.clear();
}
http://developer.garmin.com/downloads/connect-iq/monkey-c/doc/Toybox/WatchUi/DataField.html
See getBackgroundColor()