Light color inks on dark colored shirts are printed with an underbase. An underbase is a white layer of ink that the design sits on top of. This is very similar to laying down a primer or a base coat before painting a wall. It makes colors pop and keeps inks from bleeding into the garment.
For a full guide to optimizing your designs for printing with underbase, you can check out the guide 'Designing for DTG: Prep School'.