Striped Tee Shirt by ryananeily (Ryan A. Neily ryananeily@gmail.com)
released Creative Commons Zero license.

The Tee Shirt mesh was UV unwrapped and painted in GIMP.
I did a lot of backing and forthing between the two peices of software
(Blender and GIMP) to get the stripes painterly correct.

My initial desire was for a blue striped Tee, so I painted the shirt
with both blue and white for the two sets of stripes.

Once the stripes wrapped as I wanted, I looked to see how to make each set
of stripes colours changeable as per artistic requirements.

UV map image - In GIMP, I made the white stripes transparent.
The Blues remained blue.

This I loaded into my Blender UV editor....

As the "white" stripes are transparent, their colour can be chosen
with a colour wheel value in the TeeShirt material settings.
This will show through the transparent part of the image as
whatever colour you pick.

The "blue" stripes colour can be chosen in the Tee Shirt texture settings.
The colours in my version of Blender (2.79) are controlled
by the three RGB numbers. 1.0, 1.0 and 1.0 are the colour starting point
being in my image, the original blue I painted.
Any number adjustments, either positive or negative
give a colour that starts as blue.

Note: I used a cloth modifier but ended up removing it
as the result was not what I wanted.

The Tee Shirt has a Collision modifier applied.
The body of the model also has a collision modifier applied.
I have also added a pin group to pin the Tee Shirt to the shoulders
of the model.
In this case a CC0 base model.
These "pins" which hold up the tee shirt are a vertex group belonging
to the Tee Shirt mesh.