The eyedropper tool in the White Balance panel of the HUD is a handy way to automatically adjust an image’s color balance. Find a neutral area in the image, click on it, and you’ll receive Aperture’s recommendation for adjusted color correction.
But there’s an additional trick you might not know about. When you use the eyedropper, keep your finger on the mouse button and slowly move the pointer from one area to another. Aperture will update its white balance recommendation in real time. Once you discover the look you want, let go of the mouse button, and the correction is applied.
You can always undo the adjustment by clicking the arrow icon in the White Balance pane of the HUD. If you find something you really like, the use the lift and stamp tool to apply to the rest of the images in the stack.