Yes, the torches inside the walls are there to light up the exterior and create an homogeneous look and color.
A torch will light up any tile within a 15-17 radius around it, even the corner tiles, if nothing blocks the light. The exception are corners, those are ligthened up too.
I started using these tecnique to avoid darker colours in a Wall, and ended up discovering that it also makes it posible to make clearer pictures preventing the black spots and dark areas that appear when you take the screenshots from far away lvls.
As a general rule: I would say don't leave any tile in the dark. Make sure that all blocks are lightened.
Use wall torches on every lvl. One should suffice unless it is a long wall. My own personal tendency is to place one every 10 tiles but you don't need that many.
If you don't want them to be seen use decorations and bends in the wall to hide them. Keep in mind from where are you going to be looking at the structure when placing them.
If your walls are only 1 tile thick the torches in the rooms inside will lighten them up. But if you build walls that are 2+ thick the torches will need to be in the exterior or embedded. A torch on one wall will also light up a second wall in front of it.
For thick walls, rather tan embed the torches in the wall like i did in this castle it might be better to create lighting galleries inside the walls.
Sunlight: A tile is only considered to be in the sunlight if it was already like that at the beginning of the game. Any dirt or stone you may expose later, or block you build, will not be. As sunlight in this game comes in a strict vertical direction, if you build anything above them they may require torches. A weird exception is digging holes. It seems that floor tiles exposed through "dig hole" become "sunlighted". This may cause curious square patterns at the botton of moats and artificial lakes (See Brukketfell village fjord). Replace all the floor on them to prevent this.
The block at the top: Some times you may not be able to place a torch to light up a block. Particulary at the top of high roofs. Consider not placing a floor above it.
Finally: Check regularly from lvl 25 for dark spots during daytime. Anything that isn't "in the light" will become dark. Place a torch there.