Anywhere you see a color selection square in Lightroom, such as for page background color in the Print module,
you can choose a color not only from the color picker that comes up when you click on it,
but also a color from one of your photos.
How? Simply click in the main color selection area (shown above where the eyedropper is), hold the mouse button down, and drag out to your photo, either in the main window or down in the filmstrip.
You can actually select a color from anywhere on your monitor, so if you want a color that is on a web page, for example, position it next to Lightroom, then just click, hold and drag out to the web page.
You will find color options in the Print, Web and Slideshow modules, as well as in the adjustment brush and graduated filter in the Develop module.