Screen Printing is different from Direct-to-Shirt printing because this process applies the ink to the surface of the fabric, making the stiff, dried ink noticable to the touch. It also prints each color separately. So, if you have a 3 color logo, the shirt would have to be screened 3 times in order to complete the print. 

There are no shirt color restrictions with Screen Printing. Since the ink lies "on top" of the fabric, any ink color can be used on any shirt color. The only restrictions are with intricate details and half-tones.