Ok, firstly ignore the colours bit. It's blank paper that you're buying.
What you need to do with this paper is print your design onto it using an inkjet printer.
Then you take the paper with your design on and either cut it out by hand or you use a vinyl cutter machine, which is designed to cut paper aswell, to create your template.
Then you take this template and a tshirt and heat press, using a heat presser, the template wherever you want the design on the shirt.
Here's a good youtube video that shows the process:
I found this site which allows you to order the paper in smaller quantities rather than bulk's of 100.
Transfer Papers for Inkjet Printers
the difference between paper types is that some papers are better for pressing templates onto darker shirts. With the jet pro soft stretch papers you're going to get a good end result on light coloured shirts. With the papers that have 'Opaque' in their title you're going to get a better result when pressing to dark materials.
not sure about the A1 and A2 sizes though. think you would have to break your design up and do it over some A3 sheets. A3 should be enough for the front or back of a tshirt.