Any Color You Like so Long as it's Black

If you're flatting a page where panels in opposite parts of the page have the same background, here's an easy way to pick the colors for flatting the second panel (once the first panel is flatted) without having to zoom out or pan around the page to eyedrop the colors.

In Photoshop's Menubar, select Window -> Arrange -> New Window for.... (Current file)

Move this window to your second monitor, and zoom in on the flatted panel you need to eyedrop colors from. In your main monitor, zoom back to the panel you're currently flatting.

Now, when you need to eyedrop a color, just move your cursor to the window in the second display, pick up the color you need, and bucket it into your selection on the main monitor.

For even faster color picking from your reference window, eyedrop a color for foreground color, hit the 'X' key to switch foreground/background colors, and select a second color you'll need. Go back to main window, bucket, select second color area, hit 'X' again to switch colors, and fill again with second color.