I used to have chestnut hair. A week ago my sister asked if I had coloured my hair. I said no that I had coloured it several months ago and that colour was already gone. I forgot about it but few days ago, when I was at my hairdresser, she said that my hair has gotten a lot darker. Today I pulled out one of my hair strands (hope that's the right word for it) and saw that it really was very very dark. I couldn't make out if it was already black (the color I'm manifesting) or just extremely dark brown, it didn't have that red undertone that chestnut or dark chestnut hair has. My hair usually grows about 2cm a month and it looked like it had been growing for about 2-3 months. All of my hair hasn't changed color yet but the most have, the ones that are closer to my forehead are still the same color but I know that in time they will change too.
Anyway I hope that this gives inspiration to you all. :)