Those are good indicators, though you might want to try to catch them before they turn brown at all. You should also give them a squeeze, they should be fairly stiff and springy. Maybe a better tact would be to wait until they're very aromatic. Young hops will smell fresh, like new-mown grass, but mature hops should have a potent aroma more similar to what you would expect from the variety. Just pull one off and rip/cut it in half, maybe rub it in your hands, and take a whiff. If it smells ready and looks ready, it is ready.