Hi and welcome. It seems extremely unlikely a dog would have an issue with dogs of a particular colour, especially as they don't see the same way we do. Dogs can only see
two colours and cannot perceive red at all, it would just be variations on a greyish colour similar to how a tricolour or ruby or black and tan would look to them (that's why trainers suggest not using red fetch toys for dogs!). It's probably coincidence -- based on how well he was socialised generally (a stud dog should NOT be a problem dog and aggressive, especially not a cavalier!) and therefore whether these are males and seen as a threat, or females he is aggressive with as potential mates. Or, more likely, that he's simply a reactive dog due to the way in which he was kept and managed by the breeder -- especially if (as happens depressingly often with breeders) he was basically, a stud dog in her programme, used for breeding, never shown (a show cavalier could NOT act this was around other dogs or would be disqualified) and not really part of the family. It would be a bit of a red flag if she didn't warn you about this issue, as she must have seen this behaviour herself and might even be why she rehomed this particular dog because he was difficult to manage
. How well do you know her? Sadly a lot of breeders, including show breeders and even extremely well known show breeders, are less than honest about how they breed and the dogs they home.
Neutering would likely help (most responsible breeders would have neutered an ex stud before homing), as would help from a behaviourist as living with a reactive dog can be challenging. But it can be addressed and improved! I'd be looking for someone with a CPDT or APDT qualification (their websites have search functions to find local trainers/behaviourists) -- a lot of so-called self-styled dog trainers would use discredited techniques involving 'demoting' the dog and making the human the 'alpha' and so on, which generally simply increases problems and can make dogs a serious bite risk.
I've had reactive dogs that eventually settled fine in our mixed dog household but always had to be managed on walks -- never allowed off lead around unknown dogs, for example, and avoiding close encounters with other dogs. This takes extra work that not everyone wishes to take on, so might be something to consider as it will be a lifetime management task, most likely.
Ideally, you might already have a good relationship with this breeder, and she is open and honest, and unaware of this issue, and you can ask for some help with this dog and find out if he was always reactive. If you don't have that relationship and the breeder makes you uncomfortable, I'd say you are probably sensing something's not quite right about her and that, probably, she deliberately got rid of a dog she found difficult herself.
A good breeder will always be a source of advice, support and friendship for the lifetime of a dog (and beyond). One who is herself 'reactive' so to speak... not so much. Maybe she's a really supportive breeder and helpful
in which case by all means ask her for some insight and help
-- I'm just giving possible reasons why you are seeing this issue.