What Causes Armpits Redness On Switching Deodorants?

Hi there, I have switched to natural deodorants and my armpits get really red. When I try to use nothing they get really brown. When I used normal deodorants, they we're fine, but I really want to try and find an alternative. I noticed a lot of people have mentioned a rash making the switch to natural deodorant, but nothing about skin turning dark brown. What shall I do?