Ask a Dog Vet and Get Answers to Your Dog Veterinary Questions
I am sorry Killer has this lesion on his leg. There are many different things that can affect dogs skin, most of them are mild and not serious but may need some medication from your vet. There are a minority of conditions that are more serious and may require more in depth analysis.
What you describe sounds like an area of local bacterial or yeast infection. These can appear circular with a dry appearance and a halo of slightly pinker skin. Often with persistent inflammation the skin can deposit more pigment so these areas can appear darker in colour too. The treatment for this may be simple and only require the use of topical antibacterial/yeast ointment or shampoo. If more widespread it maybe necessary for oral medications too.
You could try twice daily bathing with 4% chlorhexidine solution as this can help localised skin infection. However, if getting worse a trip to the vet will be needed to ensure he correct diagnosis is made and nothing more significant is present.