As I mentioned in the title I need to change the color of the error message. By default it shows in black.
Here is my code so far :
RequiredFieldValidator rfv = new RequiredFieldValidator();
branch_name_txt.getValidators().add(rfv);
rfv.setMessage("This field is not optional.");
rfv.setIcon(new GlyphIcons().setFontAwesomeIconGlyph('\uf00d', Color.RED, 20.0));