The best and quickest way I've found is to use the Quick Mask and Pixelate>Halftone (adjust settings as necessary). Make sure to apply the filter to the Quick Mask, not the image. This will give you a pretty cool slection that you can fill with any color you want.
One of my favorite ways o make a halftone is under IMAGE change the fiile ti BITMAP. Then a window will come up and ask you how many pixles per inch and under that will be METHOD. In METHOD you select HALFTONE SCREEN click OK the another box pops up and here you can control. 1. Dots per inch 2. The angle of the dots 3. the shape of the dots