This was a style I used when I was trying to start colouring lineart. The border lineart would be black, and the inside lineart would be a darker shade of the colour inside. This was when I knew how to use clipping, etc.
I still love this style so much. I love the grey lineart, the thickness, it just... YES RUENWJS
It just didn't look good with darker colour palettes (black outfits, etc), so I stopped using this style when a friend advised me to.