They can't spoil anything as if you don't like what you have done you just clean it off, setting the colour is done by heat transfer, just takes time to do and if you are a bit of an artist you can do mottles and patterns, I had a discussion with Paul Pearce about them an we came to the same conclusion ..... just by wax crayons as they do the same thing but are far cheaper, the best one to date I did was a rug worth quite a bit and had colour run, I cleaned it reduced some of the colour and then crayoned it, heat set it then just for good luck re cleaned the area to see if there was any loss of crayon and there wasn't.
Good bit of knowledge to have.
Shaun