I've been adding new work to my Young Adult portfolio lately and to this end have taken a temporary break from my strictly analog regimen.
I start off by creating an ink wash base drawing to establish the values. You could do this in Photoshop from scratch, but I prefer doing this on paper to keep things looking organic. You could import splashes, smudges and paper texture to a digital image, but it does seem like so much work in comparison to just getting on with it on paper.
Next, I scan the drawing in and create a new layer from background. I set the properties to multiply and create a new layer below and another new layer above the 'drawing layer'. I lock the drawing layer to avoid accidents. I then add flat colours to the bottom layer using course brushes to simulate the kind of variation you would expect from real paints. I use the top layer for drawing over the drawing and adding more values as necessary.