As you can see from the examples above, slides look nice when the text color goes well with the colors in the background image. To pick a color from the image, select the text, and drag the 'text color' icon in the Colors Lab panel to the color in the image that you want to use for the text. You can see a live preview as you drag the icon.
If you want to use a color that is not in the image but goes well with the image, drag the 'main color' icon to pick a dominant color from the image. When you do that, Colors Lab will show you a selection of colors that goes well with the picked color. To pick any of those colors, select the text as before, and the drag the 'text color' icon to the suggested color you want to use. Here's how it is done:
Colors Lab comes with the free PowerPointLabs add-in and works for both PowerPoint 2010 and 2013 on Windows. You can use it to pick colors from anywhere in the screen, not just from the slide.