Not everyone has this. I personally never experienced this in either of my pregnancies. For women that get it, they might notice it in the end of their pregnancy or in the first trimester around 4-6 weeks.
This is my fourth pregnancy (1 miscarriage) and mine have never gotten darker. But my mom had four kids and she told me hers got darker before she even took her pregnancy tests.
I'M 13WKS AND I'VE NOTICED MINE GETTING DARKER ABOUT 2WKS AGO.