Dogs do not get colds such as humans do but they can get different forms which are similar. They can have Kennel Cough which is very common in dogs. While this is most commonly transferred in kennels it can also be picked up just about anywhere from dog parks to vets offices. This is very very common in dogs from shelters and the incubation period is around 2 weeks. Parainfluenza is the most common and most contagious of the dog colds, and is passed by the sneezing of the dogs. Keep in mind even with the vaccinations the dog can still get these colds.
You can bring them into the bathroom and run hot water to create steam to help break up any congestion they are having. A warm wash cloth can be used to gently clean the eyes. Make sure they are eating and drinking. You can give them some unflavored pedialyte to prevent dehydration from the dog cold. Rest and time is the best cure unless it gets worse.
You can take them to the vets office if you want to make sure it does not become a infection, though it is not required. Your vet can prescribe nose drops and cough drops as well to help clear this up. Your vet may also want to prescribe a antibiotic to help get them over this faster.
Benadryl is not very helpful in cases of colds. It helps some with allergic reactions and itching and calming the dog down in some cases but with colds it typically only makes the dog tired and slows down the healing process.
Rest and time is the best treatment............though your vet can prescribe a antibiotic or cough suppressant if it is needed.