Here is that particular stage that I was referring to.

It should be pretty clear that the light on the dog and the general background are the same color and value. There is a darkish area at the top of the canvas that I presume is a shadow cast by the "gripper" that holds my canvas on the easel.

I began by painting the canvas all one color (value #3). Covering the stark white of a canvas with a darker color is called an imprimatura. When this imprimatura was dry, I lightly sketched in the dog with a brush dipped in a darker paint, and then I began to differentiate the light and shadow on the dog.

Perhaps some of your confusion may be that you discern a slight "color or value" difference in the photo. The wet paint on the lighted areas of the dog photographed a little differently from the dry areas in the background, but they are not different.

I hope that this clarifys the process a bit for you. This was originally meant to be simple.

The "fat over lean" principle
 

"Thin" paint does not mean "lean" paint. When I did the outline drawing ("guidelines") what you see is thin paint....other than this, ALL the paint applied to this canvas was straight from the tube, thick (and lean). I paint so thickly in my bottom layers that I nearly always tend to cover and smooth out the canvas weave. For some mistaken reason or other, most people assume the opposite is true about my work.

If I had one piece of advice for a beginner, it would be (after the initial drawing) to PAINT YOUR BOTTOM LAYERS WITH VERRRRRRY THICK PAINT!

Here is an explanation of the "fat over lean" rule in indirect painting and I hope it clarifys:

In indirect painting ((Leonardo style - painting from dark to light) the artist builds up three general layers of paint and medium.

Bottom layer or layers: A color is used to block in the painting. The paint in this layer is often thinned with turpentine and a small amount of medium.

Middle layer or layers: This is where opaque colors are introduced. The medium can be thinned with 10% to 20% turpentine.

Top layer or layers: A glaze layer that modifies the opaque colors and makes the surface very rich. No thinners are added to your medium in this layer.

Paintings built in this manner follow the "fat over lean" rule. ALL PAINTING MEDIUMS ARE CONSIDERED FAT so thin your medium less as you work from the bottom layers to the top ones.

Oil paints that look shiny are FAT to begin with. And oil colors that look more matte are LEAN.

Some other painters such as Rubens who paint light to dark (and as shown in this demo) often apply thick opaque paint as the initial layer and add transparent glazes to modify that underpainting. Naturally the "fat over lean" principle applies here too.