I will continue to base my opinion on the Cockers I actually know and noone will ever convince me that Solids are more aggressive than Partis ...
Well, if I took that stance and based my opinion just on the cockers that I actually have, then I would have to say that all solids have aggression issues!
There is no point in saying that you can't be convinced of anything else but your own opinion... Mark has done some research and based on the research he did, he came to a result - whether we like that outcome or not! There is obviously a situation right now WITH THE DOGS THAT TOOK PART IN THE SURVEY that more solid coloured cockers have aggression issues in certain situations than parti coloured ones... you can't argue about that and there is no point in saying it isn't true... For that particular study and those particular dogs, it is true! And as there was quite a large number that took part from lots of areas in the UK, there might be the possibility that there is indeed a breed issue (not saying there IS, just saying there MIGHT BE).
So surely, the best approach to this issue wouldn't be to burry your head in the sand and say it doesn't exist and it's all lies, but to find out why Mark's study came to this conclusion and to do some more research and get breeders and owners (of all colours) on board to participate in that and find out whether Mark's study was just a fluke or whether there is actually an issue that needs addressing. That will be much more beneficial for the cocker as a breed than either bashing Mark for "getting it all wrong" or bashing everyone else for having the wrong opinion...

In addition, it is also important to remember that genes often don't work as isolated units in the body. In conjunction with other genes, the same gene can have different effects in different situations working with different gene combinations! So just because colour may not be an issue in one breed doesn't mean that in another breed it can be an issue due to other genes involved in certain processes. It is also a well known fact that when foxes were bred for tameness, that their colour changed (they became parti coloured)!!! This isn't to support the fact that solid coloured cockers may or may not be more aggressive, but simply to support the argument that coat colour and temperament can play together and influence each other!
Now, I have two cockers (one American, one English) and both have aggression issues. Jesse (my American, solid sable) has them due to a thyroid condition which was diagnosed when he was 18 months old... although he improved dramatically with medication, some damage to his temperament had already been done, so he isn't 100% perfect with larger dogs and puppies. (Two of his sisters, one solid, one parti, had/have the same aggression issues). Skye (my English, solid red) lost her first home at 4 months old due to her aggression issues... and my, did she have issues! They are 90% resolved and I don't know the cause. I was going to have her thyroid tested, but she is so good now, I am not sure yet whether it will be necessary. I don't know her breeding background, so don't know what her parents were like.
I would never advise someone to buy a puppy based on colour anyway regardless of the "reputation" of a colour as it were! Puppies should be aquired on the basis of temperament and health, colour should be very low on the list of priorities... So no, I wouldn't advise for or against solid cockers - whatever colour someone choses, they should chose a good breeder and then chose the puppy most suitable for their circumstances. Simples. Whether that is solid or parti coloured.

Vera
PS. Are there any other studies on coat colour and temperament connections in cockers or any other breed?
PPS. In American cockers, we tend to say that solid colours are more laid back in general whereas parti colours are more crazy and energetic... plenty of exceptions of course, but a trend that both breeders and owners seem to notice...