Did you ask if they have already had a go themselves at removing it? and if so with what? The food colouring(dyes) in soup, pasta sauces etc are so strong it makes you wonder what happen to our stomach linings when we eat them
Before you start the job, find out what they have used, if anything, quantify best expectation (low) and why. If for some reason you get a good result you will be a "miracle" worker in their eyes
If you have quantified before and the result is not good at least you made them aware of this. Good luck