I would take a very cautious position on matters such as this. Legal or not. the property owner has made their wishes clear. You saw the sign. You know they don't want you there. Go somewhere else.
I've seen similar signs where the contrast was inadequate, the size a little off, but I still know what the intent of the property owner is. No need to play games and risk a conviction after extensive litigation over whether or not the sign met the letter of the law. Just stay away and take your business elsewhere or if you must enter, disarm. Its that simple.