Here is an example where I used a 1 color raster image, along with a spot color to create what turned out to be a very nice shirt. My customer sent me a hand drawn sketch, with the beer label scanned in, and photoshopped into the line drawing. I took the image into Photoshop, made 2 copies of the image. On the first image (black screen), I erased the label, so that I had just the line drawing. For the second screen (purple), I did just the opposite. I erased everything besides the label. I then lowered the contrast/darkness, until the label was completely black. I save each image as a separate file (grayscale mode), and then opened them both in Illustrator. I converted the label to a vector with LiveTrace, but not the outline. I wanted to preserve the slight gradient of the hand drawn line. I then printed spot colors of black and purple.