Most likely the rash is really gone, but the discoloration lingers on. This is often the case after a rash in a thin-skinned area. My advice is to stop all active treatment until you see the dermatologist, and just use an unmedicated moisture lotion. When the you see skin doctor, he or she can advise you about what you need to do, if anything, other than to let the purple color take care of itself.
best.
Dr. Rockoff