This is my answer to a simple striped Tee shirt. The Tee was UV unwrapped and I tweaked the placement of the stripes in GIMP. Back and forth between Blender and GIMP.
As the “white” stripes are transparent, their colour can be chosen with a colour wheel value in the Tee Shirt 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.
If anyone has a use for a Tee shirt, the CC0 .blend file can be downloaded at Blendswap.