This is where you fill in the more solid details. The t-shirt doesn't have to have more than one color.
If you do opt for many colors, I recommend starting with the main color first. Follow with adding shadows then highlights. You might want to stretch the shirt a bit to make sure the color is getting into the grooves of the weave. Better penetration means better durability and stronger color. Yes, I said penetration. Stop giggling.
Using the paint undiluted (except with the textile medium) at this point will make the colors resistant to any "watercolor" washes you use in a later step.
Be careful when you're trying to get full coverage. If you add a little water to your color, it will be easier to press into the fibers of the shirt, but it's also more likely to bleed. It helps to touch your brush to the paper towel after loading it with paint and before painting the shirt. It blots any excess water and it shows you how runny/likely to bleed the paint is.