I did a mating with my red bitch to a blue roan dog (which is from mixed lines with almost every possible colour in the pedigree). The dog is a multi champion and has produced very nice puppies with both parti- and solid-girls. I did got four very nice puppies and the girls were mismarket when born, but both girls (now 12 weeks) are without white colour on the their face. The black girl has quite a lot of white on the chest but still it is ok. Now I have two possiblities: continue with my solid line and start to breed parties also. How facinating!
I don't know what is so bad in mixing these lines? If you get missmarked puppies you sell them to pethomes (most of them goes that way anyway.) It is a great way to freshen up the breed and maybe to make the temperaments better. It is only a colour. And that is the most unintressant thing in cocker in my point of view.

Bambi, at the age of 8 weeks (when se was born she had a very wide white stripe on her foreface and it has slowly disapeared when she has grown)
I will take a new picture of her when she comes here next weekend