Whatever you do, don't bother trying to dye them with Dylon machine dye if they get stained! We tried on a chair cover. The results were patchy and had small dark spots. The colour, supposedly a pale beige, came out a sort of orangey digestive biscuit shade, in spite of using exactly the correct weight of dye. Dylon customer services responded:
"Dylon dyes are formulated with the colour pigments to make up the over all colour, by using part of a pack of dye the formulation has not been correct.
Sofa and chair covers do have to conform to BSI standards and this does mean that they are treated with a fire retardant. The special finish used can interfere with the dyeing process. IKEA covers are normally fire and match resistant and the there will be a care label stating this.
We are sorry that you did not achieve the result you expected, unfortunately we are unable to offer a solution in this instance."
No help there, then! Since there are no warnings in the instructions that (mandatory!) fire-retardant finishes can alter the results, and the bit about partial packs is utter nonsense from a chemistry standpoint, it looks like Dylon is simply unsuitable for the task. I have seen in other forums that people HAVE done this successfully, but I think the IKEA material was changed a little while back, which could have affected results.