Well, here we are. I've done it!
So I used Dylon machine dye in tulip red and ocean blue. I put some of each aside and mixed it together to get the purple. I probably could have gotten deeper colours if I had used the right ratio of dye to weight of fabric, but I was a bit stingy and didn't want to buy two packets of each.
When I did the red (which, to be honest, has turned out slightly on the wrong side of the border between red and hot pink
) I saw all the water drain out of the washing machine and it was still bright red, and I thought surely I could have dyed something else in that. So for the blue and purple I collected the water once it had drained out of the machine (which was a difficult and messy job I can tell you) and dyed the Mothercare fitteds and also a selection of randoms like prefolds and imse vimse flannelette nappies, which as you can see have turned out a much lighter shade, but still nice and even. Except for the randoms, which have dyed a bit patchy, but that was probably because they were a bit stained to begin with.
Anyway, I'm quite pleased with the result and it should make nappy changing just a little bit more interesting.