1.

Here, notice how the area selected is well-lit and provides a realistic sample of the colour as it would be in reality? That's what we're aiming for here.

2.

The colour towards the bottom of the image here is far too light to provide an accurate match. By selecting the fringe of the light patch, the colour provided is more realistic.

3.

The image has been captured using the camera's flash, so it's key to avoid any patches of shadow. Think about how the colour appears to the eye, and simply select the area that best matches this.