Best algorithms, image processing techniques used to compare images

I need to compare a set of scanned tshirt tag labels with the main tag label image to find the differences between the two. The requirement is to identify

• picture size difference
• Character content and size differences
• color difference
• difference in positioning image content

Finally, everything should be measured and output the percentage of the image fit in the result. Please explain the appropriate algorithm and image processing techniques to build this solution.