I absolutely agree with the colors. I built out the 7 colors for frenzy first, then cut it down for puzzle mode. So frenzy looks great, but then puzzle's colors doesn't look as good.
The gameplay is a sort of the same, but there is an escalation where it gets more intense as the towers naturally build up higher. But I don't think powerups or events are a good solution for this, unless you can come up with a pretty compelling reason they would be.