Do you mean the area where you can choose from a palette of colors for your ad layout? If that's what you mean, click on the "edit palette" link indicated by the arrow in this picture: When you click there you will get the color palette that was in the main customization screen in the old version. If there is a way to randomize colors and that is what you meant, I'm afraid I'm not familiar with that.
That's kind of neat, I'd never noticed that before. For anybody else that hasn't seen this, just scroll down to the bottom of the "palettes" pulldown and select "Multiple Palettes"...then you can hold ctrl and select the ones you want.