If you use Lightroom, it's possible at some time that you wanted to use a brush to adjust colours. It's easy enough to do, or appears so.

1. Click the brush tool
2. In the drop down select Color
3. The Color Picker box shows up and you click on the colour you want

But that's not really what you want.  What you want, is to be able to select a colour from the image you are working on, not some arbitrary picker from a box or worse trying to find a place in the box that matches the colour you want to work with.

Despite lots of great UI work, I think Adobe has dropped the usability ball on this one because while it is simple, it's not obvious or well documented.  I give full credit to Rob Sylvan for teaching me how to do this.

When the colour picker box appears, click in it anywhere and hold the left mouse button down.  Without releasing the mouse, drag the eyedropper into the image.  You'll see the swatch box in the upper left of the Color Picker change to match wherever the eyedropper is stationed at any given time.  When you find the colour you want in the image, release the mouse button and the colour is loaded into the swatch and the brush is loaded with that colour.

It really is simple, but the implementation could stand to be more intuitive.  I tried to grab a screenshot but neither Skitch nor Grab would capture the image of the eyedropper, so you'll have to experiment yourself.