A merle, brindle or dilute coat is a description of how pigment is arranged on a dog, not a statement about the dog's health. The words tell you what you can see and, at best, hint at a genotype behind it. They do not tell you whether the dog hears, sees, or will stay sound, and an advertisement built on colour is usually telling you less than it seems to.
What do merle, brindle and dilute actually describe on a dog?
These are coat words. They describe pigment and pattern, not fitness.
Merle is a pattern where patches of full pigment sit against patches of reduced pigment, giving the mottled grey, blue or red look. It comes from a specific gene, and the amount of mottling varies widely between dogs. A lightly marked merle and a heavily marked one can carry the same gene.
Brindle is a pattern of dark stripes over a lighter base, laid along the body. It is common in many breeds and is not tied to the merle gene. It is a description of striping, nothing more.
Dilute is a separate mechanism. It lightens black to blue and brown to lilac, or isabella, depending on the base colour. The dog is not a different breed or a different animal; the pigment granules are packed differently. A blue dog is a black dog with a dilution gene expressed.
All three words sit in the same category as "white markings" or "long coat": useful for identifying a dog, useless for predicting health. A guide such as the one at coat colour and pattern guide treats them that way, separating what you can see from what a test can tell you. That separation is the point. If a seller uses colour as a headline, you are being handed a visual fact and asked to read a health claim into it.
Which health problems are linked to coat colour breeding?
Some, but not the ones most buyers assume, and the link is narrower than advertising suggests.
The clearest case is the merle gene. When two merle dogs are bred together, some puppies inherit two copies. Those double merle dogs are at high risk of deafness and of eye defects, including small or missing eyes and impaired vision. This is a well documented outcome, not a rare accident. It is why responsible breeders avoid merle to merle pairings, and why a litter advertised as "rare merle" deserves a direct question about what the other parent was.
Dilute coats are sometimes associated with colour dilution alopecia, a skin condition where the hair is sparse or brittle in the diluted areas. It appears in some breeds and lines more than others. It is not automatic, and a blue dog may never show it. But it is a real possibility, and it is not visible in a puppy photo.
Beyond that, the honest answer is that most health problems in Border Collies and similar working breeds have little to do with coat colour. Hip and elbow scores, eye examinations, epilepsy, and hearing are separate matters with separate tests. A dog can have a striking merle coat and excellent hips, or a plain black coat and poor ones. Colour does not sort them.
The risk is not the colour itself. The risk is a breeder who selects for colour first and treats health testing as optional, or who prices puppies by pattern. That is a business decision, and it shows up in what is missing from the paperwork, not in the coat.
What should you ask when a dog is advertised by its colour?
Ask for the things colour cannot show you, and ask in a way that produces documents rather than assurances.
Start with the parents. Ask for the results of hip and elbow scoring, an eye examination by a veterinary ophthalmologist, and any hearing testing if merle is involved. Ask whether either parent is merle, and if so, whether the pairing was merle to merle. A breeder who has done the work will have the certificates and will not mind sending them.
Ask what the colour word means in this litter. If the ad says "blue," ask whether that is a black dog with dilution or something else. If it says "merle," ask how heavily marked the parents are. These are simple questions, and vague answers are informative.
Ask about the puppy's hearing and vision directly. For merle and double merle dogs, ask whether a veterinarian has checked. For any puppy, ask what happens if a problem appears later.
Ask why the dog is being advertised this way. A working or companion dog is usually described by temperament, drive, and health, not by pattern. If colour is the first thing in the listing and the last thing in the conversation, that tells you where the seller's attention went.
Description is not genotype
A coat description and a genetic result are different kinds of information, and mixing them is where buyers get misled.
"Merle" as a word on a listing is a description of what someone sees. The genotype is what the dog actually carries, which may include a merle copy, a cryptic merle that barely shows, or a dilution gene that is invisible in a young puppy. You cannot read the genotype off a photograph, and often not off an adult dog either. A DNA test can tell you what is present. A description cannot.
The same applies to eye and nose pigment. A dog with a lot of white around the face may have less pigment in the eye or nose, and that can matter for sun sensitivity, but it is a separate observation from the coat pattern itself. Keep the categories apart: what you see, what is tested, what is examined by a vet. Each answers a different question.
If you are trying to describe a dog accurately, for a rescue listing or a breeder record, the useful habit is to write down what you observe and label it as observation. Then note separately what has been tested. That is the method a practical colour guide uses, and it is the method that keeps a description from quietly becoming a promise.
What colour does not tell you
Colour does not tell you whether a dog will herd, settle in a house, get along with children, or stay healthy into old age. It does not tell you how much exercise the dog needs or how it will handle a noisy street.
It also does not tell you what a dog is worth. A rare pattern is rare because of how genes combine, not because the dog is better. Pricing by pattern rewards the wrong selection pressure, and it is one of the clearest signals that a seller is thinking about appearance first.
What colour does tell you is limited and useful: how to identify the dog in a photo, how to describe it in a record, and sometimes which genetic questions are worth asking. Treat it as a label on a box, not a certificate inside it. When a dog is advertised by its colour, the colour is the least informative thing on the page. The health tests, the parents, and the temperament notes are where the real information sits, and those are the things to ask for before you decide anything.