When screen printing on dark fabrics, especially black, be sure the ink you're using has a good lightfastness rating. Lightfastness refers to how well a color will appear on dark or black surfaces. Review this list of Speedball ink lightfastness.
Black - Excellent
Blue - Good
Blue Denim - Good
Burgundy - Good
Green - Very good
Orange - Very good
Peacock Blue - Very good
Process Cyan - Excellent
Process Magenta - Good
Process Yellow - Good
Red - Fair/Good
Violet - Good
White - Very Good
Yellow - Fair/Good
Fluorescent Hot Pink - Poor (use on light colored shirts only)
Fluorescent Lime Green - Poor (use on light colored shirts only)Fluorescent Orange - Poor (use on light colored shirts only)Fluorescent Magenta - Poor (use on light colored shirts only)