This is really cool! What's the reason for the blue & violet colors to be raised more than others in the wave? Is it just the timing of the snapshot? As in, an

This is really cool! What's the reason for the blue & violet colors to be raised more than others in the wave? Is it just the timing of the snapshot? As in, another picture might show another color at the top of the wave? Or is the standing wave "stuck" in that position?