I don't know for sure, but I think that maybe the purple and red kit was your clubs original kit, and if so, can't be deleted. I think the only way you can get the colours you want, is to design, and buy for 6 tokens, a third kit for your team. Then you should be able to swap the new design with the red/purple kit.

I still have the original away kit I was given when I started playing (pink and grey). But I've got red/blue as my home kit, and black/orange as away. But can't delete the pink/grey.