Sweatshirts come in a wide array of weights and fabric content. Down Under Design makes all these weights available so you can choose the shirts that's just right for your needs and your wallet. Cotton is the big seller these days. Nobody today wants to iron a T-shirt. They are also more stain resistent.
Cotton is over-whelmingly the shirt of choice.
If your shirts are meant for the college market, definitely use cotton. Less magenta and the color becomes a shade of orange. Spot color is where you purple ink, or blue, or green, for example. A halftone of a royal blue will give you a lighter blue. Because of the nature of the marketplace, screen printing is often associated with T-shirts and sweats.