Is the dog having any issues from the snake bite?
Was there any pattern on the snake?
Is you dog in at the Vets office?
Does this look like the snake?
The snake above is a copperhead and would cause the signs you are seeing. For the dog to swell and have the symptoms that you have described. The other snake that would cause this is a Rattler. If it is a rattlesnake we have a problem as the swelling will increase and the site of the bite will become necrotic and the tissue start to slough off as the venom spreads.
You can see Rattlers in this picture:
With the head you described the snake is venomous and it may be wise to have your dog treated with Antivenin. I am concerned the dog needs to be put on antibiotics and supportive care. Joan
This is a link about treatment for vanomous snake bites: http://www.petplace.com/dogs/snakebites-from-a-copperhead-pit-viper-snake-venom-rattlesnake-crotalidae/page1.aspx joan
These are pictures of the venomous snakes found in Texas: http://www.texaspoison.com/snakes.asp Color variations are normal in snakes as they change color with temperatures , light and heat. The snake may also have been getting ready to shed. There are a lot of variables with color. I still feel the dog should be put on an antibiotic with supportive care. The oral benedryl may not be enough. Joan