Sometimes I feel like the queen of rashes.
I have two big Lyme rashes that appear dark/red/purple-ish. They didn't really change color to darker - they just fade away then come back when I am Herx'ing.
I have had Bartonella rashes that were dark purple, but they also just faded away and didn't really change color to darker.
I agree with sleeper and try to take a photo of the rash. I have pictures of all my strange creepy crawlies - LOL!
Maybe if its changing color - that means bacteria are dying - and that's awesome.
Everyone reacts differently to the antibiotics. I'm on Doxy also, but I take 600mg per day.
Peace and health to you.