After you read this don't hit yourself in the forehead too hard (I could have had a V8 moment).
Use the Flood Fill Tool aka the paintcan.
If you'd like to learn more about making jerseys I can email you some tutorials I wrote/helped with for other games. I used Paint Shop Pro as the basis. The tutorials are for MVP Baseball and Maximum Football and NBA Live. The principals are the same. Send me a PM with your email address. Otherwise keep asking questions if you get befuddled.