Hi Ian
My IICRC Colour Repair Instructor was quite adamant that the Dylon cotton and universal type of dyes should not be used for colour repair on wool and nylon carpets. I can't remember the reasons why, but it may be because of the long term effectiveness. You state that Ashby's encourage using these Dylon type of dyes, prepared to the closest shade. I've no experience of this type of colour matching, but it would be reasonable to presume that as with the Prism Colour Repair Kit from Chemspec and others, you would need to take into account the colour of the yarn you are dyeing, e.g. you add a blue dye to a yellow yarn to give a green colour. Again, like the Prism kit, you will need to neutralise or strip any soil/contaminants/protectors first, and add a mordaunt to set the dye.
I would agree that, in purely cost/return terms, Colour Repair may not be cost effective unless you were to market the service aggresivelly, and again I would agree that it is a valuable addition to any technician's CV. When it comes to treating 1 small bleached spot, the repair can be time consuming, but you'll still get a return of about £40 for half an hours work or so. But if you were repairing a larger stained area, producing, for example, a £100 return, you would only add about 10 minutes to your time on site as you will still only be preparing the one dye batch (per colour). Re-tufting is quite often an effective, quicker and easier route, and letting in a "patch" can be carried out successfully too on larger areas. Colour Repair can be successful on spots/stains small and large, and the real big money can be matching the colour/blending in mis-matched dye batches on new installations or light faded areas, but I've never done that sort of work.
I've never marketed my Colour Repair work particularly well, but I've seen most of the repairs I've done and re-cleaned the carpets without any problems at all. Naturally, my customers are delighted and it would be very difficult to put a monetary value on that. As a professional carpet cleaner, I would not be comfortable offering any service without suitable training.
Safe and happy cleaning:)
Ken