I'm often asked by clients, "could you work on my photo?" Recently, I was sent an image (a smaller .jpg file) that was only large enough to print 10"x10", and the color cast on the original photo was very green-yellow. This client wanted to print the photo at 20"x20" on bright aluminum, and it was important because the photo was made on a very special family vacation to Scotland.
This photo is an extreme case, and .JPG files, as opposed to RAW or .TIFF files in today's higher end cameras, are very unforgiving of strong editing. JPG files are meant as a final, compressed picture format, not as an intermediate file that's meant to be edited. It's possible to get away with corrections on JPG's, but you really have to take care not to ruin the image, especially if you are printing big, where badly printed detail is easily seen! A file is not unlimited in its potential for size and color, dynamic range, focus and the like - and you really do have to get it as right as you can in camera.
In the end, this was a success, and the "After Color Correction Photo" here was printed at 20"x20" in stunning color on Aluminum, with no JPG artifacts and sharp focus. No artificial corrections or photoshop cloning were needed or done - only a slight crop and straighten. The resulting colors are natural and authentic, and really what the client saw on the day they captured the image. I'm happy and so is the client