different color areola

Stephanie
Since the 1st week in October my areolas have been two different colors. One is pinkish ( looks Normal) but the other is clearly darker almost brown. I'm coming to the end of my cycle and the brown one keeps getting darker. I took an at home pregnancy test about a week and half ago and it was negative. I was just wondering if anyone else has experienced this or what are your thoughts on it? 
Thank you!