i don't think you need to be complaining or irate. you just go in and say you don't feel the product does the job it says it should do (assuming it does say it should do what you want it to do.) and that you'd like a refund.
I took back a faulty bourjois mascara wand (snapped off in pot) I didnt have a receipt so superdrug wouldn't refund me but gave me address of bourjois customer services and I wrote to them and they refunded me no bother.
They let you bring back opened and used makeup with a receipt in WalMart in the U.S.; there are no testers there, so they probably had to offer to return money. They should let you if you can produce a receipt and the stuff just didn't work. Plus it's not just that you bought the wrong colour and you could have seen what you were buying -- the stuff you bought didn't work over a period of time, so you couldn't have found that out in the shop.