If you prefer a different color shirt than the mockup just put that in the notes.
Print will be on a soft style t-shirt from a few options I look for: Bella + Canvas, Anvil, Next Level, and Gildan Soft Style. Depends on what is available.
Please remember that color variations may vary to what is shown in mockups. Plastisol screen prints are mixed with different colors of paint to get as close as possible to the colors shown. Each batch may print slightly different due to different mixes of paint each time.