Setting the -fx-text-fill
works for me.
See below:
if (passed) {
resultInfo.setText("Passed!");
resultInfo.setStyle("-fx-text-fill: green; -fx-font-size: 16px;");
} else {
resultInfo.setText("Failed!");
resultInfo.setStyle("-fx-text-fill: red; -fx-font-size: 16px;");
}