Discoloring of shirts when pressing labels/ shirt quality

I have recently purchased plastisol transfer tags from versatanz to label my branded shirts with. I'm using a hat press and pressing them at the required (by versatranz)temp, time and pressure. I am getting discoloration from the press on most of the tees. different colors like charcoal, teal, etc. I don't think its burning them as this is the recommended temp that everyone uses for there labels 350º for 6 sec medium pressure. the discoloration fades with time a little, but not totally. I can't have my shirts sitting in the store with these discolorations, even if they will wash out later. Shirts are canvas mens 100% cotton and next level mens 100% cotton also.
could it be the shirt quality or dye???

any help is greatly appreciated.