This week, I take that Notan design and go onto the next phase: Value study. Now, color is great and all but if you don't have a strong value structure, all of the color isn't going to save it.
So, the first thing that I do is to make a copy of my black-and-white layer in Photoshop. To do this,
1) I unlock the layer, hold down the Alt-key and then drag the layer up. As you see, I now have two layers that are exactly the same.
2) Then I use the magic wand and select the white area on the top layer.
3) Then I clear the layer using Edit->Clear.
4) Final step is to lock the top layer.
Now that that is done, this means that, when I'm painting on the top layer, it will only affect the black area... not the white area. Now, the fun starts... I select my chalk brush at 40% opacity (100% flow) and start randomly throwing down "digital paint". Like using the random strokes to create the notan design, the random paint strokes help flesh out a potential story in value.
The important part of this is to not go with a pre-conceived idea. As you randomly thrown "digital" paint around, the story will come to you. You then just flesh out what the final image will be.