When a light-colored dog falls for another light-colored dog and they breed, the resulting pups aren't necessarily going to be light-colored themselves. Genetics gets to call the shots when it comes to the color of pups that any dog has, and it's not always the parents' colors that show up in puppy litters. A color from generations back, like black, can pop up on any pup.
Two Lights Can Make a Black
Trying to grasp the genetics of dog coat colors can be difficult, but the basic explanation for two light-colored dogs producing a black pup is not as difficult: What determines the color of a puppy are his genes and how they interact with each other. Some genes hide others and some can alter the affects of others. There is a specific gene that can make a puppy black, no matter what his parents' colors are, and it happens to be a dominant gene. So if both parents are light-colored but they both carry the dominant "black" gene, chances are good that one or more of the puppies will be black or display black markings.
Predicting Puppy Color
Sometimes the physical characteristics of your parent dogs can give you a clue as to what coat colors their puppies will have. For instance if both parents have light-colored coats but also have light-colored eyes and pink or other pale-colored skin on their noses, you're seeing indications that they may not carry darker color genes. Alternatively, if either parent is light-colored with a dark nose and dark eyes, that parent is likely to be carrying genes that will result in puppies that are chocolate-colored or darker. Using this approach to determine what color your dog's pups will be isn't an exact science. Either parent can be carrying a gene that he or she inherited from generations ago that might suddenly push its way to the front of the line and color one or more puppies a shade that neither of the parents displays.
If you simply must know for sure what color of puppies your dogs are likely to produce, talk to your vet about DNA testing. A specific test will tell you if your light-colored parents carry the forms of the genes -- also known as alleles -- that will give them black or brown puppies.
When It Won't Happen
Sometimes genetics dictates that light-colored dogs will never have black puppies. This is the case with purebred dogs whose breed standards assert the only color to be white, as with Westies and bichon frise. These dogs, if truly purebred, are always white even though their noses and eyes are black.