Derek is correct, there are two specific dyes in Irn Bru, a yellow and a red, the only thing that works is high heat.............but..........you could lose additional colour from the fibres.
There are claims from Kirby, who sell a product which they claim will remove Irn Bru and if used with high heat you will get a limited result, but the product, is just a similar product to many others, which most suppliers offer, to remove, red wine, fruit drinks, etc.
If you offer to attempt a high heat treatment ( domestic iron through clean white towel ) make absolutely sure that you warn the client of the possibility of colour loss.
In 90% of cases I suggest..........insurance claim !
good luck
robbie
