Making a shirt..? Iron on picture?

I have this iron on paper stuff that you put in the printer and print out a picture you want, and then iron it onto a shirt. So, I was gonna do that. But I went out and bought a dark purple shirt before reading the directions, and when I came home and read them it said "only use on light colored products." What do you think the worse this is that could happen? I don't wanna mess it up, but I really want a certain picture on it. I need it for Friday and I'm making it tomorrow.